fixes #2458 add polyfill for str_starts_with function

Introduces a compatibility implementation of str_starts_with for environments where it is not available. Updates common.inc.php to include the polyfill if the function does not exist.

include/php_compat/str_starts_with.php

@@ -0,0 +1,8 @@
+<?php
+if (!function_exists('str_starts_with'))
+{
+  function str_starts_with(string $haystack, string $needle): bool
+  {
+    return strlen($needle) === 0 || strpos($haystack, $needle) === 0;
+  }
+}