PHP prevent external form submission

[PHP] code

$referer = isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : NULL;
$host = $_SERVER['HTTP_HOST'];
echo 'Submitted address:'. $referer;
echo '';
echo 'This site's domain name: '.$host;
echo '';
echo substr($referer,7,strlen($host));
 if(substr($referer,7,strlen($host)) != $host){
 echo 'illegal operation';
}else{
 echo 'normal operation';
}

PHP