feat: add php solution to lc problem: No.1137 (doocs#935)

Author: Qiu-IT

solution/1100-1199/1137.N-th Tribonacci Number/README.md

@@ -137,6 +137,29 @@ var tribonacci = function (n) {
 };
 ```
 
+### **PHP**
+
+```php
+class Solution {
+    /**
+     * @param Integer $n
+     * @return Integer
+     */
+    function tribonacci($n) {
+        if ($n == 0) {
+            return 0;
+        } else if ($n == 1 || $n == 2) {
+            return 1;
+        }
+        $dp = [0, 1, 1];
+        for ($i = 3; $i <= $n; $i++) {
+            $dp[$i] = $dp[$i - 1] + $dp[$i - 2] + $dp[$i - 3];
+        }
+        return $dp[$n];
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1100-1199/1137.N-th Tribonacci Number/README_EN.md

@@ -130,6 +130,29 @@ var tribonacci = function (n) {
 };
 ```
 
+### **PHP**
+
+```php
+class Solution {
+    /**
+     * @param Integer $n
+     * @return Integer
+     */
+    function tribonacci($n) {
+        if ($n == 0) {
+            return 0;
+        } else if ($n == 1 || $n == 2) {
+            return 1;
+        }
+        $dp = [0, 1, 1];
+        for ($i = 3; $i <= $n; $i++) {
+            $dp[$i] = $dp[$i - 1] + $dp[$i - 2] + $dp[$i - 3];
+        }
+        return $dp[$n];
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1100-1199/1137.N-th Tribonacci Number/Solution.php

@@ -0,0 +1,18 @@
+class Solution {
+    /**
+     * @param Integer $n
+     * @return Integer
+     */
+    function tribonacci($n) {
+        if ($n == 0) {
+            return 0;
+        } else if ($n == 1 || $n == 2) {
+            return 1;
+        }
+        $dp = [0, 1, 1];
+        for ($i = 3; $i <= $n; $i++) {
+            $dp[$i] = $dp[$i - 1] + $dp[$i - 2] + $dp[$i - 3];
+        }
+        return $dp[$n];
+    }
+}