PHP_SELF in a mysql query

I am attempting to gather information from a mysql database where the files_filename field equals PHP_SELF (the current document name):

$query = "select * from files where files_filename = $PHP_SELF";

This isn't working does anyone have any ideas?
Thanks

Try putting quotation marks ( ' and ' ) around the $PHP_SELF

I have tried that but it takes it as literal. In other words, if files_filename had an entry $PHP_SELF in the field it would output it.

My apologies. I thought I had tried that but I guess I was wrong - it works now thanks.