PHP Newbie Database Records?

PHP Newbie, I'm trying to find out if a username already appears in my database. I have two users in my table, one is colin so when I enter a username of colin I would normally under ColdFusion anyway just to something like: <cfif $query_qUsername.RecordCount GT 0> User already exists do something else, like throw the user back an error. </cfif> My Query works $query_qUsername = "SELECT * FROM users WHERE username = '$username'"; I've tried a number of different things here and everything either comes back with nothing, or throws an error. $num_rows = mysql_num_rows($query_qUsername); echo $num_rows; // this comes back with nothing, I was expecting a numeric of 1 exit; Regards Col