feat: add php solution to lc problem: No.0451 (#1002)

Author: Qiu-IT

solution/0400-0499/0451.Sort Characters By Frequency/README.md

@@ -178,6 +178,28 @@ impl Solution {
 }
 ```
 
+### **PHP**
+
+```php
+ class Solution {
+    /**
+     * @param String $s
+     * @return String
+     */
+    function frequencySort($s) {
+        for ($i = 0; $i < strlen($s); $i++) {
+            $hashtable[$s[$i]] += 1;
+        }
+        arsort($hashtable);
+        $keys = array_keys($hashtable);
+        for ($j = 0; $j < count($keys); $j++) {
+            $rs = $rs.str_repeat($keys[$j], $hashtable[$keys[$j]]);
+        }
+        return $rs;
+    }
+};
+```
+
 ### **...**
 
 ```

solution/0400-0499/0451.Sort Characters By Frequency/README_EN.md

@@ -162,6 +162,28 @@ impl Solution {
 }
 ```
 
+### **PHP**
+
+```php
+ class Solution {
+    /**
+     * @param String $s
+     * @return String
+     */
+    function frequencySort($s) {
+        for ($i = 0; $i < strlen($s); $i++) {
+            $hashtable[$s[$i]] += 1;
+        }
+        arsort($hashtable);
+        $keys = array_keys($hashtable);
+        for ($j = 0; $j < count($keys); $j++) {
+            $rs = $rs.str_repeat($keys[$j], $hashtable[$keys[$j]]);
+        }
+        return $rs;
+    }
+};
+```
+
 ### **...**
 
 ```

solution/0400-0499/0451.Sort Characters By Frequency/Solution.php

@@ -0,0 +1,17 @@
+ class Solution {
+    /**
+     * @param String $s
+     * @return String
+     */
+    function frequencySort($s) {
+        for ($i = 0; $i < strlen($s); $i++) {
+            $hashtable[$s[$i]] += 1;
+        }
+        arsort($hashtable);
+        $keys = array_keys($hashtable);
+        for ($j = 0; $j < count($keys); $j++) {
+            $rs = $rs.str_repeat($keys[$j], $hashtable[$keys[$j]]);
+        }
+        return $rs;
+    }
+};