To find out if your username exists in the database, you need to make a SELECT query to the database asking for any records with $username in them.

I would suggest that you read some of the tutorials here on DBI:

• Reading from a database
• Tricks with DBI

(I'm refraining from answering the question directly because I believe you'll find learning the answer from the tutorials more rewarding in the long term.) :)

I would also suggest that you look into using placeholders in your query. Right now, you're interpolating the strings '$username' etc. into your insert statement. While this will work most of the time, someone could enter "T'Pau" as a username, and your insertion statement would be:

...Values ('T'Pau'...
[download]

which would then be a database error, since the quotation would end after the T. At very least, I'd change it to the following:

 
$query = qq(insert into members (username, password, email, name, url,
+ date) Values (?, ?, ?, ?, ?, ?));
$sth = $dbh->prepare($query);
$sth->execute($username, $password, $email, $name, $url, $time) or die
+ "Error in execution: $DBI::errstr";
[download]

You can learn more about this in the abovementioned tutorials.

stephen

Update: Added explanation of indirect answer.