Username Check problem

[Undrium]

I've searched through the forum for help but can't really solve it that way. What's the problem with this username check function :

$usercheck = mysql_query("SELECT username from users WHERE username=$username");

  if ($usercheck){
  header( "Location: http://www.XXX.com/register.php?error=exists" );
 	}

Username is a text and I do connect to the database before this. Any help?

[thorpe]

if ($usercheck) {
  if (mysql_num_rows($usercheck) > 0) { 
    header( "Location: http://www.XXX.com/register.php?error=exists" ); 
  }
}

[Undrium]

$usercheck = mysql_query("SELECT username from users WHERE username='$username'");


  if (mysql_num_rows($usercheck) > 0) { 
  header( "Location: http://www.XXX.com/register.php?error=exists" ); 
  }

This one worked like a charm. Thanks for the kickstart. 🙂