Whitelisted domains validation for UltimateMember plugin

add_action( 'um_before_new_user_register', 'require_whitelisted_email_for_signup' );

function require_whitelisted_email_for_signup( $args ) {
	$user_email       = $args['user_email'];
	$approved_domains = array( 'yahoo.com', 'gmail.com', 'hotmail.com' );
	$email_approved   = false;
	foreach ( $approved_domains as $domain ) {
		$prefixed_domain = '@' . $domain;
		if ( str_ends_with( $user_email, $prefixed_domain ) ) {
			$email_approved = true;
			break;
		}
	}
	if ( ! $email_approved ) {
		exit( wp_safe_redirect( add_query_arg( 'err', 'blocked_domain' ) ) );
	}
}

function str_ends_with( $haystack, $needle ) {
	$needle_position          = strripos( $haystack, $needle, - 1 ); // -1 = Look at the exact end
	$needle_expected_position = strlen( $haystack ) - strlen( $needle );    // length of the haystack minus length of the needle = beginning of the needle in the haystack
	return ( $needle_position === $needle_expected_position );
}

Loving it! 😄