添加(0232.用栈实现队列.md)：PHP版本

problems/0232.用栈实现队列.md

@@ -496,5 +496,49 @@ void myQueueFree(MyQueue* obj) {
 }
 ```
 
+PHP:
+```php
+// SplStack 类通过使用一个双向链表来提供栈的主要功能。[PHP 5 >= 5.3.0, PHP 7, PHP 8]
+// https://www.php.net/manual/zh/class.splstack.php
+class MyQueue {
+    // 双栈模拟队列：In栈存储数据；Out栈辅助处理
+    private $stackIn;
+    private $stackOut;
+    
+    function __construct() {
+        $this->stackIn = new SplStack();
+        $this->stackOut = new SplStack();
+    }
+
+    // In: 1 2 3  <= push  
+    function push($x) {
+        $this->stackIn->push($x);
+    }
+
+    function pop() {
+        $this->peek();
+        return $this->stackOut->pop();
+    }
+
+    function peek() {
+        if($this->stackOut->isEmpty()){
+            $this->shift();
+        }
+        return $this->stackOut->top();
+    }
+
+    function empty() {
+        return $this->stackOut->isEmpty() && $this->stackIn->isEmpty();
+    }
+
+    // 如果Out栈为空，把In栈数据压入Out栈
+    // In: 1 2 3 => pop    push => 1 2 3 :Out  
+    private function shift(){
+        while(!$this->stackIn->isEmpty()){
+            $this->stackOut->push($this->stackIn->pop());
+        }
+    }
+}
+```
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