unique user id

[Anon]

I am trying to get my site to check a database (mysql) for usernames that have already been used. I have this little script but it doesn't seem to recognise when an already used name is re-entered into the registration page.
Anyone got any ideas??

mysql_connect (localhost, username, password);

mysql_select_db (dbname);

$result = mysql_query ("SELECT * FROM tablename
WHERE USER_NAME = '$USER_NAME'
");

if ($row = mysql_fetch_array($result))
{
print ("<b>Error:</b> The user name <b>");
print ("$USER_NAME");
print ("</b> has been taken. Go back and enter a new user name.");
print ("<p>");
$verify = "bad";
}
else
{
print ("User name: ");
print ("$USER_NAME");
}

[Anon]

Firstly, the mysql functions normally return values, so make sure you've got variables to handle them, the query function needs to have a connection variable, also I always put die functions on the end of my mySQL functions cause I might have a stupid moment and I want to check.

Also the mysql_fetch_array function is a bit superflous, $row isn't used but is taking memory, try

if (mysql_num_rows($result))