htmlentities extended?? any function which excludes some tags??

hi guys
i want my visitor to be able to put <a> tag in their comment and no other html tags.. now i'm using htmlentities which is removing all the HTML tags.. now i want to know whether there is any function which will work as htmlentities but will take some tags as parameters to make exception for those tags...

 

take a look at the strip_tags function. i believe that one allows for exceptions.

 

$str=strip_tags($str, '<a>');