二叉树的递归遍历 add php version

Author: alanx15a2

problems/二叉树的递归遍历.md

@@ -671,6 +671,62 @@ public void Traversal(TreeNode cur, IList<int> res)
 }
 ```
 
+### PHP
+```php
+// 144.前序遍历
+function preorderTraversal($root) {
+    $output = [];
+    $this->traversal($root, $output);
+    return $output;
+}
+
+function traversal($root, array &$output) {
+    if ($root->val === null) {
+        return;
+    }
+
+    $output[] = $root->val;
+    $this->traversal($root->left, $output);
+    $this->traversal($root->right, $output);
+}
+```
+```php
+// 94.中序遍历
+function inorderTraversal($root) {
+    $output = [];
+    $this->traversal($root, $output);
+    return $output;
+}
+
+function traversal($root, array &$output) {
+    if ($root->val === null) {
+        return;
+    }
+
+    $this->traversal($root->left, $output);
+    $output[] = $root->val;
+    $this->traversal($root->right, $output);
+}
+```
+```php
+// 145.后序遍历
+function postorderTraversal($root) {
+    $output = [];
+    $this->traversal($root, $output);
+    return $output;
+}
+
+function traversal($root, array &$output) {
+    if ($root->val === null) {
+        return;
+    }
+
+    $this->traversal($root->left, $output);
+    $this->traversal($root->right, $output);
+    $output[] = $root->val;
+}
+```
+
 
 <p align="center">
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">