添加 0018.四数之和 PHP版本

problems/0018.四数之和.md

@@ -310,6 +310,49 @@ var fourSum = function(nums, target) {
 };
 ```
 
+PHP:
+```php
+class Solution {
+    /**
+     * @param Integer[] $nums
+     * @param Integer $target
+     * @return Integer[][]
+     */
+    function fourSum($nums, $target) {
+        $res = [];
+        sort($nums);
+        for ($i = 0; $i < count($nums); $i++) {
+            if ($i > 0 && $nums[$i] == $nums[$i - 1]) {
+                continue;
+            }
+            for ($j = $i + 1; $j < count($nums); $j++) {
+                if ($j > $i + 1 && $nums[$j] == $nums[$j - 1]) {
+                    continue;
+                }
+                $left = $j + 1;
+                $right = count($nums) - 1;
+                while ($left < $right) {
+                    $sum = $nums[$i] + $nums[$j] + $nums[$left] + $nums[$right];
+                    if ($sum < $target) {
+                        $left++;
+                    }
+                    else if ($sum > $target) {
+                        $right--;
+                    }
+                    else {
+                        $res[] = [$nums[$i], $nums[$j], $nums[$left], $nums[$right]];
+                        while ($left < $right && $nums[$left] == $nums[$left+1]) $left++;
+                        while ($left < $right && $nums[$right] == $nums[$right-1]) $right--;
+                        $left++;
+                        $right--;
+                    }
+                }
+            }
+        }
+        return $res;
+    }
+}
+```
 
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 * 作者微信：[程序员Carl](https://mp.weixin.qq.com/s/b66DFkOp8OOxdZC_xLZxfw)