Send custom refereal code to registration in WordPress - wordpress

Question

I'm passing a referral code that I'm saving to wp_usertmeta with a custom field added in functions.php
So far, so good!
Link to wp register: wp-login.php?action=register&ref=2
I save the referral code with:
<?php $ref = $_GET['ref']?>
It works BUT, if i enter a username thats already taken i get the standard error message from WordPress that it's taken.
When this happens the URL reloads to: wp-login.php?action=register and i cant use $_GET['ref']
Is there any alternative way to do this?

Answer 1

The easiest way to solve this would be to save the $_GET['ref'] value to a cookie or the PHP session. Once the user is registered, you can refer to the cookie/session to save to the database using the user_register action hook. Once it is saved, you should clear this value if it is a cookie unless it is needed for something else to reduce the size of each request.
//set the cookie
if ( isset( $_GET['ref'] ) ){
setcookie( "ref", $_GET['ref'] );
}
Then save the value using the action hook
add_action( 'user_register', 'my_user_register' );
function my_user_register( $user_id ){
if ( isset( $_COOKIE['ref'] ) ){
// save the ref to the user meta
update_user_meta( $user_id, 'ref', $_COOKIE['ref'] );
// delete the cookie
setcookie( "ref", null, -1 );
}
}