problems with ctype_alnum() function

I am using this function

if(!isset($username) or strlen($username) <3){

$msg=$msg."User id should be =3 or more than 3 char length<BR>";

$status= "NOTOK";}					



if(!ctype_alnum($username)){

$msg=$msg."User id should contain alphanumeric  chars only<BR>";

$status= "NOTOK";}	

At one point it was working just fine I haven't changed anything and then suddenly it started giving me the error message:

[quote]User id should contain alphanumeric chars only[/quote]

No matter what username a enter it still gives me that error message.
I have tried all combinations of usernames.
alpha and alphanumeric but the same thing happens.
Any ideas please any suggestions ?
Thanks