AMP shim support for Gravity Forms submissions

amp-gravity-forms-submission-shim.php

<?php
/**
 * AMP Gravity Forms Submission Shim plugin bootstrap.
 *
 * @package   Google\AMP_Gravity_Forms_Submission_Shim
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2020 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Gravity Forms Submission Shim
 * Plugin URI: https://gist.github.com/westonruter/b2554073461707a88057bb86b6b603d6
 * Description: Plugin which shims AMP compatibility for Gravity Forms by fixing redirection upon submission and ensuring HTML5 form validation is used for required fields.
 * Version: 0.1.4
 * Author: Weston Ruter
 * Author URI: https://weston.ruter.net/
 * License: GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * Gist Plugin URI: https://gist.github.com/westonruter/b2554073461707a88057bb86b6b603d6
 */

// Shim support for Gravity Form submissions on AMP pages.
add_action( 'gform_post_submission', function() {
	$location = null;
	foreach ( headers_list() as $header ) {
		if ( preg_match( '/^Location:\s*(.+)/i', $header, $matches ) ) {
			$location = $matches[1];
			break;
		}
	}

	if ( $location ) {
		header_remove( 'Location' );
		header( "AMP-Redirect-To: $location" );
		header( 'AMP-Access-Control-Allow-Source-Origin: ' . home_url() );
		header( 'Access-Control-Expose-Headers: AMP-Redirect-To, AMP-Access-Control-Allow-Source-Origin' );
		wp_send_json(
			[
				'message'     => __( 'Redirecting…', 'amp' ),
				'redirecting' => true, // Make sure that the submit-success doesn't get styled as success since redirection _could_ be to error page.
			],
			200
		);
	}
} );

// Use client-side validation UI instead of showing validation errors after page reload. Props Rahul Bansal at rtCamp.
add_filter( 'gform_field_content', function( $content, $field ) {
	if ( $field->isRequired ) {
		return str_replace( 'aria-required=', 'required=', $content );
	}
	return $content;
}, 10, 2 );

Related: https://gist.github.com/swissspidy/86e3a50ec5c0f7d46ec4de43824e23a0
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