Gerando ID's para usuários no ato do registro

/** Gerar ID's para usuários **/
add_action( 'user_register', 'my_on_user_register' );
function my_on_user_register( $user_id ) {
    $unique_id = 6395730800131050 + $user_id;
    update_user_meta( $user_id, 'my_unique_id', $unique_id );
}
//display decal code on profile
add_action('show_user_profile', 'my_extra_user_profile_fields');
add_action('edit_user_profile', 'my_extra_user_profile_fields');
function my_extra_user_profile_fields($user){
     $unique_number = get_the_author_meta('my_unique_id', $user->ID);
 ?>
     <h3><?php _e('Decal Info'); ?></h3>

     <table class="form-table"> 
         <tr> 
             <th><label for="address"><?php _e('Code:'); ?></label></th> -->
             <td><?php echo $unique_number; ?></td> 
         </tr> 
     </table> 
 <?php
 }

// Dar echo na variável que contém o número
add_action('num_card_print', 'num_card', 10);
 function num_card() {
 	$cartao_id = get_user_meta('my_unique_id', 'my_unique_id', true);
 	echo $cartao_id;
 }

//send email with decal code to admin on new user
 function registration_email_alert($user_id) {
     $message = strip_tags($_POST['user_login']). ' - ' . strip_tags($_POST['user_email']) . ' Decal Code: ' . strip_tags($user_id['my_unique_id']) . ' Has Registered To Your Website';
     wp_mail( '[email protected]', 'New User Has Been Registered', $message );
 }
 add_action('user_register', 'registration_email_alert');
 //add decal code to user list
 function new_modify_user_table( $column ) {
     $column['my_unique_id'] = 'Decal Code';
     return $column;
 }
 add_filter( 'manage_users_columns', 'new_modify_user_table' );

 function new_modify_user_table_row( $val, $column_name, $user_id ) {
     $user = get_userdata( $user_id );
     switch ($column_name) {
         case 'my_unique_id' :
             return get_the_author_meta( 'my_unique_id', $user_id );
             break;
         default:
     }
     return $return;
}
 add_filter( 'manage_users_custom_column', 'new_modify_user_table_row', 10, 3 );

veja este numero : 11 caracteres se for maior da error
$unique_id = 6395730800131050 + $user_id;

o corretor seria:
$unique_id = rand(999,999999) + $user_id;