feat: add php solution to lc problem: No.1491 (doocs#988)

Author: Qiu-IT

solution/1400-1499/1491.Average Salary Excluding the Minimum and Maximum Salary/README.md

@@ -199,6 +199,27 @@ double average(int* salary, int salarySize) {
 }
 ```
 
+### **PHP**
+
+```php
+class Solution {
+    /**
+     * @param Integer[] $salary
+     * @return Float
+     */
+    function average($salary) {
+        $max = $sum = 0;
+        $min = 10 ** 6;
+        for ($i = 0; $i < count($salary); $i++) {
+            $min = min($min, $salary[$i]);
+            $max = max($max, $salary[$i]);
+            $sum += $salary[$i];
+        }
+        return ($sum - $max - $min) / (count($salary) - 2);
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1400-1499/1491.Average Salary Excluding the Minimum and Maximum Salary/README_EN.md

@@ -170,6 +170,27 @@ double average(int* salary, int salarySize) {
 }
 ```
 
+### **PHP**
+
+```php
+class Solution {
+    /**
+     * @param Integer[] $salary
+     * @return Float
+     */
+    function average($salary) {
+        $max = $sum = 0;
+        $min = 10 ** 6;
+        for ($i = 0; $i < count($salary); $i++) {
+            $min = min($min, $salary[$i]);
+            $max = max($max, $salary[$i]);
+            $sum += $salary[$i];
+        }
+        return ($sum - $max - $min) / (count($salary) - 2);
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1400-1499/1491.Average Salary Excluding the Minimum and Maximum Salary/Solution.php

@@ -0,0 +1,16 @@
+class Solution {
+    /**
+     * @param Integer[] $salary
+     * @return Float
+     */
+    function average($salary) {
+        $max = $sum = 0;
+        $min = 10 ** 6;
+        for ($i = 0; $i < count($salary); $i++) {
+            $min = min($min, $salary[$i]);
+            $max = max($max, $salary[$i]);
+            $sum += $salary[$i];
+        }
+        return ($sum - $max - $min) / (count($salary) - 2);
+    }
+}