feat: add php solution to lc problem: No.0643 (doocs#975)

Author: Qiu-IT

solution/0600-0699/0643.Maximum Average Subarray I/README.md

@@ -121,6 +121,30 @@ impl Solution {
 }
 ```
 
+### **PHP**
+
+```php
+class Solution {
+    /**
+     * @param Integer[] $nums
+     * @param Integer $k
+     * @return Float
+     */
+    function findMaxAverage($nums, $k) {
+        $sum = 0;
+        for ($i = 0; $i < $k; $i++) {
+            $sum += $nums[$i];
+        }
+        $max = $sum;
+        for ($j = $k; $j < count($nums); $j++) {
+            $sum = $sum - $nums[$j - $k] + $nums[$j];
+            $max = max($max, $sum);
+        }
+        return $max / $k;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/0600-0699/0643.Maximum Average Subarray I/README_EN.md

@@ -109,6 +109,30 @@ impl Solution {
 }
 ```
 
+### **PHP**
+
+```php
+class Solution {
+    /**
+     * @param Integer[] $nums
+     * @param Integer $k
+     * @return Float
+     */
+    function findMaxAverage($nums, $k) {
+        $sum = 0;
+        for ($i = 0; $i < $k; $i++) {
+            $sum += $nums[$i];
+        }
+        $max = $sum;
+        for ($j = $k; $j < count($nums); $j++) {
+            $sum = $sum - $nums[$j - $k] + $nums[$j];
+            $max = max($max, $sum);
+        }
+        return $max / $k;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/0600-0699/0643.Maximum Average Subarray I/Solution.php

@@ -0,0 +1,19 @@
+class Solution {
+    /**
+     * @param Integer[] $nums
+     * @param Integer $k
+     * @return Float
+     */
+    function findMaxAverage($nums, $k) {
+        $sum = 0;
+        for ($i = 0; $i < $k; $i++) {
+            $sum += $nums[$i];
+        }
+        $max = $sum;
+        for ($j = $k; $j < count($nums); $j++) {
+            $sum = $sum - $nums[$j - $k] + $nums[$j];
+            $max = max($max, $sum);
+        }
+        return $max / $k;
+    }
+}