Facebook regsitration plugin - async validation (email) - php

Question

We have an existing membership site setup in php/MySQL, and are looking to integrate Facebook registration and login. The login and registration forms have been imported, and are hooked up to php/MySQL code which creates new users in the database. However, in order that we can prevent a single user having multiple logons (eg. one site logon, one Facebook logon), we need to check the email address of the user before adding a new line to the members database table.
We can do this through php/MySQL code, but that gets needlessly complex when you have to cater for all the permutation of cross-site/FB membership, plus we do not want the user to connect to the FB registration app without becoming a site member. A better solution would be to prevent the user from registering under an email address which is already present in the database (rather directing them to obtain a new password, and update connect their facebook profile to the existing profile). We have been looking for weeks for a solution, and have consulted the Facebook Developers Async Validation (http://developers.facebook.com/docs/plugins/registration/advanced/), as well as the stackexchange forum, but have not been able to implement a solution which validates the email address against the database, and prevents/allows registration accordingly.
The code we have so far is
<fb:registration redirect-uri="http://www.mysite.com/register.php"
fields='[{"name":"name"},{"name":"email"}
{"name":"username","description":"Username","type":"text"}]'
onvalidate="validate_async"></fb:registration>
<script src="https://code.jquery.com/jquery-1.4.4.min.js"></script>
<script>
function validate_async(form, cb) {
$.getJSON('http://www.mysite.com/register_check.php/' + form.email + '?callback=?',
function(response) {
if (response.error) {
// Username isn't taken, let the form submit
cb();
}
cb({username: 'That email is taken'});
});
}
</script>
register_check.php
//connect to the database
include 'mysql_connect.php'
$email = $_GET('email');
$data = array();
$sqlCommand = "SELECT * FROM members WHERE email='$email'";
$query = mysql_query($sqlCommand) or die (mysql_error());
$num_rows = mysql_num_rows($query);
if($num_rows>0){
$email_check = $row['email'];
$data['email'] = $email_check;
} else {
$data['error'] = "true";
}
echo json_encode($data);
Unfortunately, this process is not working at all. The registration form simply hangs endlessly, without validating nor passing the information to the database. We have done considerable research into how to properly code getJSON and the server-side php, but have come up against brick wall, so would very much appreciate any help or advice on this issue.

Answer 1

I hope this isn't too late, but after facing the same problem I found out the registration plugin doesn't send with the form object the preset details (name, email, gender, birthdate) -- only sends any custom fields. Quite dumb imo but could have an advantage or two.
Solution was to override the description of the email so as to trick it into thinking it's custom (which it would be) -- disadvantage being your regular checks will not be checked against (valid email et al) so you'll need to take care of those manually.

Answer 2

This may be late, but:
Change your JSON address to
http://www.mysite.com/register_check.php?email=' + form.email +
You were using facebook's check for existing user. Change:
cb({username: 'That email is taken'});
to:
cb({email: 'That email is taken'});
You want error message to appear next to email field.