feat: add php solution to lc problem: No.0268 (doocs#928)

Author: Qiu-IT

solution/0200-0299/0268.Missing Number/README.md

@@ -203,6 +203,25 @@ var missingNumber = function (nums) {
 };
 ```
 
+### **PHP**
+
+```php
+class Solution {
+    /**
+     * @param Integer[] $nums
+     * @return Integer
+     */
+    function missingNumber($nums) {
+        $n = count($nums);
+        $sumN = ($n + 1) * $n / 2;
+        for ($i = 0; $i < $n; $i++) {
+            $sumN -= $nums[$i];
+        }
+        return $sumN;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/0200-0299/0268.Missing Number/README_EN.md

@@ -173,6 +173,25 @@ var missingNumber = function (nums) {
 };
 ```
 
+### **PHP**
+
+```php
+class Solution {
+    /**
+     * @param Integer[] $nums
+     * @return Integer
+     */
+    function missingNumber($nums) {
+        $n = count($nums);
+        $sumN = ($n + 1) * $n / 2;
+        for ($i = 0; $i < $n; $i++) {
+            $sumN -= $nums[$i];
+        }
+        return $sumN;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/0200-0299/0268.Missing Number/Solution.php

@@ -0,0 +1,14 @@
+class Solution {
+    /**
+     * @param Integer[] $nums
+     * @return Integer
+     */
+    function missingNumber($nums) {
+        $n = count($nums);
+        $sumN = ($n + 1) * $n / 2;
+        for ($i = 0; $i < $n; $i++) {
+            $sumN -= $nums[$i];
+        }
+        return $sumN;
+    }
+}