checking if username exists

troinfo

Hi,
I\'m new to php and I\'m having a problem checking if a username already exist in my database. I\'m using JS to validate the form and after that is all done I wish to check for existing usernames before going to the next page. Then just have a $msg say the name is taken.

here my code:

if ($submit) {

$db_name = \"*******\";

$table_name = \"user_info\";

$connection = @mysql_connect(\"***\", \"***\", \"\") or die(\"Couldn\'t connect.\");

$db = @mysql_select_db($db_name, $connection) or die(\"Couldn\'t select database.\");

$sql = \"select username from user_info where username =\'$username\'\";

$result = @($sql,$connection) or die(\"Couldn\'t execute query 1.\");

if ($exists) {

$msg = \"that name alrealdy exists\";

}else {

exit;

}

any help would be great.
Thanks,
Troy

[deleted]

Looks pretty good up until this point:

if ($exists) {
$msg = \"that name alrealdy exists\";

I would change that to:

if(mysql_num_rows($result) > 0) {
echo "Name exists.";

Hope that helps,
Matt Wade
codewalkers.com

troinfo

thanks for your reply 🙂

it is working now i just needed to modify the JS. Thanks Matt 🙂

One last question : the if ($submit) statement, does this prevent a connection to the data base until the submit button is pressed?

Anon

If $submit is passed only via a submit button, then no it will not since the parser will not have heard of $submit (unless $submit has been set by submitting the page) and so all the code between the braces will be ignored.

crislewis

One thing that occurs to me is that if you are using this routine to check if a username exists before adding a new user who wnats the dame username you might end up in trouble. Theorectically, on a busy site or with just bad timing two new users might attempt to use the same username in quick succession. There is a finite gap between the select (to determine a username is already in use) and the insert of a new record and another user can insert in this gap and then the second insert will fail on duplicate key.

The simple and correct way around this is always to insert and check the result. If it is OK then its all done. If mysql reports an error check for duplicate key and ask for a different username.

Cheers,

Cris

mattw

That is good advice, but I would think that the possiblity of two users trying to register with the same username at the exact "finite" gap are pretty unlikely. In fact, I am willing to wager that I win the lottery first 🙂

Anyway, it is good advice, you can never be too careful...

Matt Wade
codewalkers.com

crislewis

I'll take that bet. $100 to you if you win the jackpot for your state lottery before you get a duplicate key error from your user table.

What you forget is that the odds are always against you in the lottery but sod's law will come up trumps when it's something bad that's not supposed to happen!

Cheers,

Cris