$handle = fopen("dosyaadi.php", "r");
$new_data = '';
if ($handle) {
         while (($line = fgets($handle)) !== false) {
               $new_data .= $line;
               if(strpos($line, '<head') !== false){
                   $new_data .= $_POST['formelementadi'];
               }
         }
        fclose($handle);
        file_put_contents('dosyaadi.php', $new_data);
}