Security Validation

Every time i implement any type of security i get an error on load.
PHP Fatal error: Call to undefined function mlang_message() in BLAHBLAHBLAH\include\phpfunctions.php on line 1161
This does not occur with no login. Any help would be appreciated.
{

$endpos=strpos($str,"}",$pos);

if($endpos===false)

{

$xt->report_error("Page is broken");

return;

}

$tag = trim(substr($str,$pos+15,$endpos-$pos-15));

$start=$endpos+1;

Line 1161: echo htmlspecialchars(mlang_message($tag));

}