Merge pull request #1393 from fmtvar/0344

添加（0344.反转字符串.md）：PHP版本

problems/0344.反转字符串.md

@@ -266,6 +266,38 @@ public class Solution
     }
 }
 ```
+
+
+PHP：
+```php
+// 双指针
+// 一：
+function reverseString(&$s) {
+    $left = 0;
+    $right = count($s)-1;
+    while($left<$right){
+        $temp = $s[$left];
+        $s[$left] = $s[$right];
+        $s[$right] = $temp;
+        $left++;
+        $right--;
+    }
+}
+
+// 二：
+function reverseString(&$s) {
+    $this->reverse($s,0,count($s)-1);
+}
+// 按指定位置交换元素
+function reverse(&$s, $start, $end) {
+    for ($i = $start, $j = $end; $i < $j; $i++, $j--) {
+        $tmp = $s[$i];
+        $s[$i] = $s[$j];
+        $s[$j] = $tmp;
+    }
+    }
+```
+
 Scala:
 ```scala
 object Solution {