right i have a basic sign-up script but how can i get it to...
check if the username is already in use
then if not
creat the user
if so return to error page
heres my basic script

<?php
include 'includes/db.inc.php';

$username = $_POST['username'];
$password = $_POST['password'];
$first_name = $_POST['first_name'];
$last_name = $_POST['last_name'];
$address1 = $_POST['address1'];
$address2 = $_POST['address2'];
$address3 = $_POST['address3'];
$county = $_POST['county'];
$postcode = $_POST['postcode'];
$country = $_POST['contry'];
$tel = $_POST['tel'];
$fax = $_POST['fax'];
$mob = $_POST['mob'];
$email = $_POST['email'];
$web = $_POST['web'];

$a_user = strtoupper("$username");
$salt = substr($a_user, 0, 2);
$crypted_password = crypt($password, $salt);

mysql_connect($hostName, $userName, $passWord);

@mysql_select_db($databaseName) or die( "Unable to select database");
$query = "INSERT INTO members 
            SET members_username = '$a_user',
                members_password = '$crypted_password',
                members_firstname = '$first_name',
                members_lastname = '$last_name',
                members_address1 = '$address1',
                members_address2 = '$address2',
                members_address3 = '$address3',
                members_stateprovince = '$county',
                members_zippostcode = '$postcode',
                members_country = '$country',
                members_tel = '$tel',
                members_fax = '$fax',
                members_mob = '$mob',
                members_email = '$email', 
                members_web = '$web'";

@mysql_select_db($databaseName) or die( "Unable to select database");
$query2 = "INSERT INTO page 
            SET page_admin = '$username'";

@mysql_select_db($databaseName) or die( "Unable to select database");
$query3 = "INSERT INTO freds 
            SET fred_admin = '$username.1'";

@mysql_select_db($databaseName) or die( "Unable to select database");
$query4 = "INSERT INTO freds 
            SET fred_admin = '$username.2'";

@mysql_select_db($databaseName) or die( "Unable to select database");
$query5 = "INSERT INTO freds 
            SET fred_admin = '$username.3'";

@mysql_select_db($databaseName) or die( "Unable to select database");
$query6 = "INSERT INTO freds 
            SET fred_admin = '$username.4'";

@mysql_select_db($databaseName) or die( "Unable to select database");
$query7 = "INSERT INTO freds 
            SET fred_admin = '$username.5'";

mysql_query($query);
mysql_query($query2);
mysql_query($query3);
mysql_query($query4);
mysql_query($query5);
mysql_query($query6);
mysql_query($query7);
mysql_close();

header("Location: index.php");
?>