WP log in/out plugin question

I need a plug in that can check a user account against a database to see if a user is allowed to still log in based on criteria outside WordPress. I tried writing it this way, but it doesn’t work. Any suggestions?

function testTransaction () {
	global $current_user;
	get_currentuserinfo();
        //if is admin
	if ( $current_user->user_level >= 4 )
	{
	  return;
	} else {
		$c = mysql_connect(/*login stuff*/);
		mysql_select_db(/*blah blah*/,$c);
		$query = "query";
		$result = mysql_query($query,$c) or die (mysql_error($c));
		$row = mysql_fetch_assoc($result);

		//Testing code left out here//
                if(test)
		 {
			return;
		} else {
			wp_logout();
			return;
		}
	}

}
add_action('wp_head', 'testTransaction');

Thing is when I run this nothing happens. No test is run, and everyone logs in fine, regardless of whether or not they should pass the test. Thoughts?