<?php
function ABC($a)
{
    $output = str_replace("script", "", $a);
    $output = str_replace("alert", "", $output);
    $output = str_replace("img", "", $output);
    $output = str_replace("onerror", "", $output);    
    return $output;
}
 $a = $_GET['a'];
 $b = ABC($a);
 echo $b;
?>