diff --git a/framework/base/ErrorHandler.php b/framework/base/ErrorHandler.php
index a62529199f..7f1afa89af 100644
--- a/framework/base/ErrorHandler.php
+++ b/framework/base/ErrorHandler.php
@@ -347,7 +347,7 @@ abstract class ErrorHandler extends Component
         if ($exception instanceof Exception) {
             $message = "Exception ({$exception->getName()})";
         } elseif ($exception instanceof ErrorException) {
-            $message = "{$exception->getName()}";
+            $message = (string)$exception->getName();
         } else {
             $message = 'Exception';
         }
diff --git a/framework/console/UnknownCommandException.php b/framework/console/UnknownCommandException.php
index 7b8ca07cd8..95e336b639 100644
--- a/framework/console/UnknownCommandException.php
+++ b/framework/console/UnknownCommandException.php
@@ -74,8 +74,7 @@ class UnknownCommandException extends Exception
         list($helpController, $actionID) = $help;
 
         $availableActions = [];
-        $commands = $helpController->getCommands();
-        foreach ($commands as $command) {
+        foreach ($helpController->getCommands() as $command) {
             $result = $this->application->createController($command);
             if ($result === false) {
                 continue;
diff --git a/framework/console/controllers/AssetController.php b/framework/console/controllers/AssetController.php
index 187cdfdfb2..4f2e492dce 100644
--- a/framework/console/controllers/AssetController.php
+++ b/framework/console/controllers/AssetController.php
@@ -646,7 +646,7 @@ EOD;
             $fullMatch = $matches[0];
             $inputUrl = $matches[1];
 
-            if (strpos($inputUrl, '/') === 0 || strpos($inputUrl, '#') === 0 || preg_match('/^https?:\/\//i', $inputUrl) || preg_match('/^data:/i', $inputUrl)) {
+            if (strncmp($inputUrl, '/', 1) === 0 || strncmp($inputUrl, '#', 1) === 0 || preg_match('/^https?:\/\//i', $inputUrl) || preg_match('/^data:/i', $inputUrl)) {
                 return $fullMatch;
             }
             if ($inputFileRelativePathParts === $outputFileRelativePathParts) {
diff --git a/framework/console/controllers/CacheController.php b/framework/console/controllers/CacheController.php
index 2a43ce6547..abb9293a88 100644
--- a/framework/console/controllers/CacheController.php
+++ b/framework/console/controllers/CacheController.php
@@ -300,6 +300,6 @@ class CacheController extends Controller
      */
     private function canBeFlushed($className)
     {
-        return !is_a($className, ApcCache::className(), true) || php_sapi_name() !== 'cli';
+        return !is_a($className, ApcCache::className(), true) || PHP_SAPI !== 'cli';
     }
 }
diff --git a/framework/console/controllers/HelpController.php b/framework/console/controllers/HelpController.php
index 4a72d48138..5bb5040ff7 100644
--- a/framework/console/controllers/HelpController.php
+++ b/framework/console/controllers/HelpController.php
@@ -75,8 +75,7 @@ class HelpController extends Controller
      */
     public function actionList()
     {
-        $commands = $this->getCommandDescriptions();
-        foreach ($commands as $command => $description) {
+        foreach ($this->getCommandDescriptions() as $command => $description) {
             $result = Yii::$app->createController($command);
             if ($result === false || !($result[0] instanceof Controller)) {
                 continue;
@@ -116,15 +115,13 @@ class HelpController extends Controller
             return;
         }
 
-        $arguments = $controller->getActionArgsHelp($action);
-        foreach ($arguments as $argument => $help) {
+        foreach ($controller->getActionArgsHelp($action) as $argument => $help) {
             $description = str_replace("\n", '', addcslashes($help['comment'], ':')) ?: $argument;
             $this->stdout($argument . ':' . $description . "\n");
         }
 
         $this->stdout("\n");
-        $options = $controller->getActionOptionsHelp($action);
-        foreach ($options as $argument => $help) {
+        foreach ($controller->getActionOptionsHelp($action) as $argument => $help) {
             $description = str_replace("\n", '', addcslashes($help['comment'], ':'));
             $this->stdout('--' . $argument . ($description ? ':' . $description : '') . "\n");
         }
@@ -158,8 +155,7 @@ class HelpController extends Controller
             $this->stdout($scriptName . ' ' . $this->ansiFormat($action->getUniqueId(), Console::FG_YELLOW));
         }
 
-        $args = $controller->getActionArgsHelp($action);
-        foreach ($args as $name => $arg) {
+        foreach ($controller->getActionArgsHelp($action) as $name => $arg) {
             if ($arg['required']) {
                 $this->stdout(' <' . $name . '>', Console::FG_CYAN);
             } else {
@@ -215,7 +211,7 @@ class HelpController extends Controller
         $class = new \ReflectionClass($controller);
         foreach ($class->getMethods() as $method) {
             $name = $method->getName();
-            if ($name !== 'actions' && $method->isPublic() && !$method->isStatic() && strpos($name, 'action') === 0) {
+            if ($name !== 'actions' && $method->isPublic() && !$method->isStatic() && strncmp($name, 'action', 6) === 0) {
                 $actions[] = Inflector::camel2id(substr($name, 6), '-', true);
             }
         }
@@ -535,8 +531,7 @@ class HelpController extends Controller
      */
     protected function formatOptionAliases($controller, $option)
     {
-        $aliases = $controller->optionAliases();
-        foreach ($aliases as $name => $value) {
+        foreach ($controller->optionAliases() as $name => $value) {
             if ($value === $option) {
                 return ', -' . $name;
             }
diff --git a/framework/console/controllers/MessageController.php b/framework/console/controllers/MessageController.php
index 70d38a95c5..77705da7cb 100644
--- a/framework/console/controllers/MessageController.php
+++ b/framework/console/controllers/MessageController.php
@@ -804,7 +804,7 @@ EOD;
                 // add obsolete unused messages
                 foreach ($existingMessages as $message => $translation) {
                     if (!$removeUnused && !isset($merged[$category . chr(4) . $message]) && !isset($todos[$category . chr(4) . $message])) {
-                        if (!$markUnused || (!empty($translation) && ((substr($translation, 0, 2) === '@@' && substr($translation, -2) === '@@')))) {
+                        if (!$markUnused || (!empty($translation) && (substr($translation, 0, 2) === '@@' && substr($translation, -2) === '@@'))) {
                             $todos[$category . chr(4) . $message] = $translation;
                         } else {
                             $todos[$category . chr(4) . $message] = '@@' . $translation . '@@';
diff --git a/framework/console/controllers/MigrateController.php b/framework/console/controllers/MigrateController.php
index a3d0d34a67..5211834f0f 100644
--- a/framework/console/controllers/MigrateController.php
+++ b/framework/console/controllers/MigrateController.php
@@ -483,7 +483,7 @@ class MigrateController extends BaseMigrateController
             $property = array_shift($chunks);
 
             foreach ($chunks as $i => &$chunk) {
-                if (strpos($chunk, 'foreignKey') === 0) {
+                if (strncmp($chunk, 'foreignKey', 10) === 0) {
                     preg_match('/foreignKey\((\w*)\s?(\w*)\)/', $chunk, $matches);
                     $foreignKeys[$property] = [
                         'table' => isset($matches[1])
diff --git a/framework/data/Sort.php b/framework/data/Sort.php
index 7f0326630d..7ee59d2282 100644
--- a/framework/data/Sort.php
+++ b/framework/data/Sort.php
@@ -257,8 +257,7 @@ class Sort extends BaseObject
                 $params = $request instanceof Request ? $request->getQueryParams() : [];
             }
             if (isset($params[$this->sortParam])) {
-                $attributes = $this->parseSortParam($params[$this->sortParam]);
-                foreach ($attributes as $attribute) {
+                foreach ($this->parseSortParam($params[$this->sortParam]) as $attribute) {
                     $descending = false;
                     if (strncmp($attribute, '-', 1) === 0) {
                         $descending = true;
diff --git a/framework/db/ActiveQuery.php b/framework/db/ActiveQuery.php
index fdd5837222..46086fbbab 100644
--- a/framework/db/ActiveQuery.php
+++ b/framework/db/ActiveQuery.php
@@ -762,7 +762,7 @@ class ActiveQuery extends Query implements ActiveQueryInterface
      */
     public function viaTable($tableName, $link, callable $callable = null)
     {
-        $modelClass = $this->primaryModel !== null ? get_class($this->primaryModel) : get_class();
+        $modelClass = $this->primaryModel !== null ? get_class($this->primaryModel) : __CLASS__;
 
         $relation = new self($modelClass, [
             'from' => [$tableName],
diff --git a/framework/db/Query.php b/framework/db/Query.php
index 3bc6333ba9..79e81b0d75 100644
--- a/framework/db/Query.php
+++ b/framework/db/Query.php
@@ -610,6 +610,8 @@ PATTERN;
         } elseif (!is_array($columns)) {
             $columns = preg_split('/\s*,\s*/', trim($columns), -1, PREG_SPLIT_NO_EMPTY);
         }
+        // this sequantial assignment is needed in order to make sure select is being reset
+        // before using getUniqueColumns() that checks it
         $this->select = [];
         $this->select = $this->getUniqueColumns($columns);
         $this->selectOption = $option;
diff --git a/framework/db/pgsql/JsonExpressionBuilder.php b/framework/db/pgsql/JsonExpressionBuilder.php
index bd64e89833..e8ea47452d 100644
--- a/framework/db/pgsql/JsonExpressionBuilder.php
+++ b/framework/db/pgsql/JsonExpressionBuilder.php
@@ -39,7 +39,7 @@ class JsonExpressionBuilder implements ExpressionBuilderInterface
             return "($sql)" . $this->getTypecast($expression);
         }
         if ($value instanceof ArrayExpression) {
-            $placeholder = "array_to_json(" . $this->queryBuilder->buildExpression($value, $params) . ")";
+            $placeholder = 'array_to_json(' . $this->queryBuilder->buildExpression($value, $params) . ')';
         } else {
             $placeholder = $this->queryBuilder->bindParam(Json::encode($value), $params);
         }
diff --git a/framework/db/pgsql/Schema.php b/framework/db/pgsql/Schema.php
index 6559424554..462e8ba1a7 100644
--- a/framework/db/pgsql/Schema.php
+++ b/framework/db/pgsql/Schema.php
@@ -438,8 +438,7 @@ SQL;
     {
         $uniqueIndexes = [];
 
-        $rows = $this->getUniqueIndexInformation($table);
-        foreach ($rows as $row) {
+        foreach ($this->getUniqueIndexInformation($table) as $row) {
             if ($this->db->slavePdo->getAttribute(\PDO::ATTR_CASE) === \PDO::CASE_UPPER) {
                 $row = array_change_key_case($row, CASE_LOWER);
             }
@@ -546,7 +545,7 @@ SQL;
                     $column->defaultValue = new Expression($column->defaultValue);
                 } elseif ($column->type === 'boolean') {
                     $column->defaultValue = ($column->defaultValue === 'true');
-                } elseif (stripos($column->dbType, 'bit') === 0 || stripos($column->dbType, 'varbit') === 0) {
+                } elseif (strncasecmp($column->dbType, 'bit', 3) === 0 || strncasecmp($column->dbType, 'varbit', 6) === 0) {
                     $column->defaultValue = bindec(trim($column->defaultValue, 'B\''));
                 } elseif (preg_match("/^'(.*?)'::/", $column->defaultValue, $matches)) {
                     $column->defaultValue = $column->phpTypecast($matches[1]);
diff --git a/framework/db/sqlite/Schema.php b/framework/db/sqlite/Schema.php
index d6dd66b0c4..92749147e6 100644
--- a/framework/db/sqlite/Schema.php
+++ b/framework/db/sqlite/Schema.php
@@ -451,6 +451,6 @@ class Schema extends \yii\db\Schema implements ConstraintFinderInterface
      */
     private function isSystemIdentifier($identifier)
     {
-        return strpos($identifier, 'sqlite_') === 0;
+        return strncmp($identifier, 'sqlite_', 7) === 0;
     }
 }
diff --git a/framework/filters/Cors.php b/framework/filters/Cors.php
index 584f504f04..f00da60305 100644
--- a/framework/filters/Cors.php
+++ b/framework/filters/Cors.php
@@ -139,8 +139,7 @@ class Cors extends ActionFilter
     public function extractHeaders()
     {
         $headers = [];
-        $requestHeaders = array_keys($this->cors);
-        foreach ($requestHeaders as $headerField) {
+        foreach (array_keys($this->cors) as $headerField) {
             $serverField = $this->headerizeToPhp($headerField);
             $headerData = isset($_SERVER[$serverField]) ? $_SERVER[$serverField] : null;
             if ($headerData !== null) {
diff --git a/framework/helpers/BaseArrayHelper.php b/framework/helpers/BaseArrayHelper.php
index e40929dfe4..d72303e1a7 100644
--- a/framework/helpers/BaseArrayHelper.php
+++ b/framework/helpers/BaseArrayHelper.php
@@ -119,8 +119,7 @@ class BaseArrayHelper
         $args = func_get_args();
         $res = array_shift($args);
         while (!empty($args)) {
-            $next = array_shift($args);
-            foreach ($next as $k => $v) {
+            foreach (array_shift($args) as $k => $v) {
                 if ($v instanceof UnsetArrayValue) {
                     unset($res[$k]);
                 } elseif ($v instanceof ReplaceArrayValue) {
diff --git a/framework/helpers/BaseStringHelper.php b/framework/helpers/BaseStringHelper.php
index f7469c14cc..524f1d729f 100644
--- a/framework/helpers/BaseStringHelper.php
+++ b/framework/helpers/BaseStringHelper.php
@@ -310,7 +310,7 @@ class BaseStringHelper
      */
     public static function normalizeNumber($value)
     {
-        $value = "$value";
+        $value = (string)$value;
 
         $localeInfo = localeconv();
         $decimalSeparator = isset($localeInfo['decimal_point']) ? $localeInfo['decimal_point'] : null;
diff --git a/framework/helpers/BaseVarDumper.php b/framework/helpers/BaseVarDumper.php
index 5354e269cb..5cb57ce2ec 100644
--- a/framework/helpers/BaseVarDumper.php
+++ b/framework/helpers/BaseVarDumper.php
@@ -72,10 +72,10 @@ class BaseVarDumper
                 self::$_output .= $var ? 'true' : 'false';
                 break;
             case 'integer':
-                self::$_output .= "$var";
+                self::$_output .= (string)$var;
                 break;
             case 'double':
-                self::$_output .= "$var";
+                self::$_output .= (string)$var;
                 break;
             case 'string':
                 self::$_output .= "'" . addslashes($var) . "'";
diff --git a/framework/i18n/Formatter.php b/framework/i18n/Formatter.php
index cc1929db9b..76e03001ad 100644
--- a/framework/i18n/Formatter.php
+++ b/framework/i18n/Formatter.php
@@ -1005,7 +1005,7 @@ class Formatter extends Component
             $zeroDateTime = (new DateTime())->setTimestamp(0);
             $valueDateTime = (new DateTime())->setTimestamp(abs($value));
             $interval = $valueDateTime->diff($zeroDateTime);
-        } elseif (strpos($value, 'P-') === 0) {
+        } elseif (strncmp($value, 'P-', 2) === 0) {
             $interval = new DateInterval('P' . substr($value, 2));
             $isNegative = true;
         } else {
diff --git a/framework/log/migrations/m141106_185632_log_init.php b/framework/log/migrations/m141106_185632_log_init.php
index 40d0bf794b..4a8b5b54c1 100644
--- a/framework/log/migrations/m141106_185632_log_init.php
+++ b/framework/log/migrations/m141106_185632_log_init.php
@@ -60,8 +60,7 @@ class m141106_185632_log_init extends Migration
 
     public function up()
     {
-        $targets = $this->getDbTargets();
-        foreach ($targets as $target) {
+        foreach ($this->getDbTargets() as $target) {
             $this->db = $target->db;
 
             $tableOptions = null;
@@ -86,8 +85,7 @@ class m141106_185632_log_init extends Migration
 
     public function down()
     {
-        $targets = $this->getDbTargets();
-        foreach ($targets as $target) {
+        foreach ($this->getDbTargets() as $target) {
             $this->db = $target->db;
 
             $this->dropTable($target->logTable);
diff --git a/framework/mutex/OracleMutex.php b/framework/mutex/OracleMutex.php
index a9cc203e99..6edfdee758 100644
--- a/framework/mutex/OracleMutex.php
+++ b/framework/mutex/OracleMutex.php
@@ -67,7 +67,7 @@ class OracleMutex extends DbMutex
     public function init()
     {
         parent::init();
-        if (strpos($this->db->driverName, 'oci') !== 0 && strpos($this->db->driverName, 'odbc') !== 0) {
+        if (strncmp($this->db->driverName, 'oci', 3) !== 0 && strncmp($this->db->driverName, 'odbc', 4) !== 0) {
             throw new InvalidConfigException('In order to use OracleMutex connection must be configured to use Oracle database.');
         }
     }
diff --git a/framework/web/ErrorHandler.php b/framework/web/ErrorHandler.php
index f1bf66c189..bdf4f99a7d 100644
--- a/framework/web/ErrorHandler.php
+++ b/framework/web/ErrorHandler.php
@@ -232,7 +232,7 @@ class ErrorHandler extends \yii\base\ErrorHandler
      */
     protected function getTypeUrl($class, $method)
     {
-        if (strpos($class, 'yii\\') !== 0) {
+        if (strncmp($class, 'yii\\', 4) !== 0) {
             return null;
         }
 
diff --git a/framework/web/Request.php b/framework/web/Request.php
index 7173999135..75a2d89741 100644
--- a/framework/web/Request.php
+++ b/framework/web/Request.php
@@ -809,7 +809,7 @@ class Request extends \yii\base\Request
             } elseif (isset($_SERVER['PHP_SELF']) && ($pos = strpos($_SERVER['PHP_SELF'], '/' . $scriptName)) !== false) {
                 $this->_scriptUrl = substr($_SERVER['SCRIPT_NAME'], 0, $pos) . '/' . $scriptName;
             } elseif (!empty($_SERVER['DOCUMENT_ROOT']) && strpos($scriptFile, $_SERVER['DOCUMENT_ROOT']) === 0) {
-                $this->_scriptUrl = str_replace('\\', '/', str_replace($_SERVER['DOCUMENT_ROOT'], '', $scriptFile));
+                $this->_scriptUrl = str_replace([$_SERVER['DOCUMENT_ROOT'], '\\'], ['', '/'], $scriptFile);
             } else {
                 throw new InvalidConfigException('Unable to determine the entry script URL.');
             }
@@ -1196,7 +1196,7 @@ class Request extends \yii\base\Request
          * RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
          */
         $auth_token = $this->getHeaders()->get('HTTP_AUTHORIZATION') ?: $this->getHeaders()->get('REDIRECT_HTTP_AUTHORIZATION');
-        if ($auth_token !== null && stripos($auth_token, 'basic') === 0) {
+        if ($auth_token !== null && strncasecmp($auth_token, 'basic', 5) === 0) {
             $parts = array_map(function ($value) {
                 return strlen($value) === 0 ? null : $value;
             }, explode(':', base64_decode(mb_substr($auth_token, 6)), 2));
diff --git a/framework/web/Response.php b/framework/web/Response.php
index d4aa1cdbda..387012ab00 100644
--- a/framework/web/Response.php
+++ b/framework/web/Response.php
@@ -366,8 +366,7 @@ class Response extends \yii\base\Response
             throw new HeadersAlreadySentException($file, $line);
         }
         if ($this->_headers) {
-            $headers = $this->getHeaders();
-            foreach ($headers as $name => $values) {
+            foreach ($this->getHeaders() as $name => $values) {
                 $name = str_replace(' ', '-', ucwords(str_replace('-', ' ', $name)));
                 // set replace for first occurrence of header but false afterwards to allow multiple
                 $replace = true;
@@ -767,7 +766,11 @@ class Response extends \yii\base\Response
      */
     protected function getDispositionHeaderValue($disposition, $attachmentName)
     {
-        $fallbackName = str_replace('"', '\\"', str_replace(['%', '/', '\\'], '_', Inflector::transliterate($attachmentName, Inflector::TRANSLITERATE_LOOSE)));
+        $fallbackName = str_replace(
+            ['%', '/', '\\', '"'],
+            ['_', '_', '_', '\\"'],
+            Inflector::transliterate($attachmentName, Inflector::TRANSLITERATE_LOOSE)
+        );
         $utfName = rawurlencode(str_replace(['%', '/', '\\'], '', $attachmentName));
 
         $dispositionHeader = "{$disposition}; filename=\"{$fallbackName}\"";
@@ -841,7 +844,7 @@ class Response extends \yii\base\Response
             $url[0] = '/' . ltrim($url[0], '/');
         }
         $url = Url::to($url);
-        if (strpos($url, '/') === 0 && strpos($url, '//') !== 0) {
+        if (strncmp($url, '/', 1) === 0 && strncmp($url, '//', 2) !== 0) {
             $url = Yii::$app->getRequest()->getHostInfo() . $url;
         }
 
diff --git a/framework/web/UrlManager.php b/framework/web/UrlManager.php
index 5ad3da1eae..aa5f01fa31 100644
--- a/framework/web/UrlManager.php
+++ b/framework/web/UrlManager.php
@@ -437,7 +437,7 @@ class UrlManager extends Component
                     }
 
                     return $url . $baseUrl . $anchor;
-                } elseif (strpos($url, '//') === 0) {
+                } elseif (strncmp($url, '//', 2) === 0) {
                     if ($baseUrl !== '' && ($pos = strpos($url, '/', 2)) !== false) {
                         return substr($url, 0, $pos) . $baseUrl . substr($url, $pos) . $anchor;
                     }
@@ -546,7 +546,7 @@ class UrlManager extends Component
         $url = $this->createUrl($params);
         if (strpos($url, '://') === false) {
             $hostInfo = $this->getHostInfo();
-            if (strpos($url, '//') === 0) {
+            if (strncmp($url, '//', 2) === 0) {
                 $url = substr($hostInfo, 0, strpos($hostInfo, '://')) . ':' . $url;
             } else {
                 $url = $hostInfo . $url;
diff --git a/framework/web/UrlRule.php b/framework/web/UrlRule.php
index eca688d405..2f0582350f 100644
--- a/framework/web/UrlRule.php
+++ b/framework/web/UrlRule.php
@@ -239,7 +239,7 @@ class UrlRule extends BaseObject implements UrlRuleInterface
             } else {
                 $this->host = $this->pattern;
             }
-        } elseif (strpos($this->pattern, '//') === 0) {
+        } elseif (strncmp($this->pattern, '//', 2) === 0) {
             if (($pos2 = strpos($this->pattern, '/', 2)) !== false) {
                 $this->host = substr($this->pattern, 0, $pos2);
             } else {
@@ -336,7 +336,7 @@ class UrlRule extends BaseObject implements UrlRuleInterface
         $this->pattern = '#^' . trim(strtr($this->_template, $tr), '/') . '$#u';
 
         // if host starts with relative scheme, then insert pattern to match any
-        if (strpos($this->host, '//') === 0) {
+        if (strncmp($this->host, '//', 2) === 0) {
             $this->pattern = substr_replace($this->pattern, '[\w]+://', 2, 0);
         }
 
@@ -593,7 +593,7 @@ class UrlRule extends BaseObject implements UrlRuleInterface
      */
     private function trimSlashes($string)
     {
-        if (strpos($string, '//') === 0) {
+        if (strncmp($string, '//', 2) === 0) {
             return '//' . trim($string, '/');
         }