NOTE: This answer looks irrelevant to the OP's question after further analysis of the added/updated function in the question. Im leaving this answer in place and creating a new answer for historical use, since part of the question seems applicable.
You could use something like the following to create user meta when users are added to the user table. Put this in the functions.php file of your theme.
function myplugin_registration_save( $user_id ) { if ( isset( $_POST['first_name'] ) ) update_user_meta( $user_id, 'your_key', // name of key $your_value // value of your key ); } add_action( 'user_register', 'myplugin_registration_save', 10, 1 ); To retrieve this data for later use use the function:
get_user_meta($user_id, 'your_key', $single); More info on user_register hook More info on update_user_meta() More info on get_user_meta()