I have a script that allows users to become a member of my website, and this piece of code is used to check that there isn't already a user registered in the database with the same email address:

PHP Code:

// check that duplicate email does not exist in tblUser
    $sql = mysql_query ("SELECT userEmail FROM tblUser
                         WHERE userEmail = '$userEmail'");

    if ($sql) {
        $error[] = 'userEmailDup';
      } 


The thing is, no matter what email address I type in, even if there is not duplicate in the database... it still produces an error!

I assume that the problem has to be in the code above, am I right?

Try something like:

// check that duplicate email does not exist in tblUser
$sql = mysql_query ("SELECT userEmail FROM tblUser WHERE userEmail = '$userEmail' LIMIT 0,1");
$sql_count = mysql_num_rows($sql);

// If Email exists, produce an error.
if ($sql_count == 1) {
        $error[] = 'userEmailDup';
}

The problem is that your query is successful, so if($sql) always returns true even if your query returns an empty result. Use COUNT.

PHP Code:

$result = mysql_query("SELECT COUNT(*) FROM tblUser WHERE userEmail = '$userEmail';");
$count = mysql_fetch_array($result, MYSQL_NUM);
$count = $count[0];

if($count > 0)
{
     $error[] = 'userEmailDup';
} 


Ahhh thanks!