⚠️ Do not use. This is an anti-pattern! See https://github.com/ampproject/amphtml/issues/21912#issuecomment-617995565

amp-webp-img-wrapper.php

<?php
/**
 * AMP WebP Img Wrapper
 *
 * @package   AMP_WebP_Img_Wrapper
 * @author    Weston Ruter, Google
 * @link      https://gist.github.com/westonruter/332abdb2adefc6b204ad6fcc1beecedf
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP WebP Img Wrapper
 * Plugin URI: https://gist.github.com/westonruter/332abdb2adefc6b204ad6fcc1beecedf
 * Description: Wrap all generated amp-img elements with another amp-img that points to a WebP.
 * Version: 0.1
 * Author: Weston Ruter, Google
 * 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/332abdb2adefc6b204ad6fcc1beecedf
 */

namespace AMP_WebP_Img_Wrapper;

/**
 * Filter sanitizers.
 *
 * @param array $sanitizers Sanitizers.
 * @return array Sanitizers.
 */
function filter_sanitizers( $sanitizers ) {
	require_once __DIR__ . '/class-sanitizer.php';

	return array_merge(
		$sanitizers,
		[
			__NAMESPACE__ . '\Sanitizer' => [],
		]
	);
}
add_filter( 'amp_content_sanitizers', __NAMESPACE__ . '\filter_sanitizers' );

class-sanitizer.php

<?php
/**
 * Sanitizer
 *
 * @package AMP_WebP_Img_Wrapper
 */

namespace AMP_WebP_Img_Wrapper;

use AMP_Base_Sanitizer;
use DOMElement;
use DOMXPath;

/**
 * Class Sanitizer
 */
class Sanitizer extends AMP_Base_Sanitizer {

	/**
	 * Sanitize.
	 */
	public function sanitize() {
		$xpath = new DOMXPath( $this->dom );

		/**
		 * Element.
		 *
		 * @var DOMElement $amp_img
		 */
		foreach ( $xpath->query( '//amp-img' ) as $amp_img ) {

			$src         = $amp_img->getAttribute( 'src' );
			$webp_src    = $this->rewrite_webp_img_url( $src );
			$srcset      = $amp_img->getAttribute( 'srcset' );
			$webp_srcset = $this->rewrite_webp_img_url( $srcset );

			if ( $src !== $webp_src || $srcset !== $webp_srcset ) {
				$webp_amp_img = $amp_img->cloneNode( false );
				if ( $webp_src ) {
					$webp_amp_img->setAttribute( 'src', $webp_src );
				}
				if ( $webp_srcset ) {
					$webp_amp_img->setAttribute( 'srcset', $webp_srcset );
				}
				$amp_img->parentNode->replaceChild( $webp_amp_img, $amp_img );
				$webp_amp_img->appendChild( $amp_img );
				$amp_img->setAttribute( 'fallback', '' );
			}
		}
	}

	/**
	 * Rewrite non-WebP img URL to be WebP.
	 *
	 * @param string $src Image src for non-WebP image.
	 * @return string WebP image URL.
	 */
	private function rewrite_webp_img_url( $src ) {
		return preg_replace(
			'/(?<=\.)(jpg|jpeg|png|gif)(?=$|\?|,|\s)/',
			'webp',
			(string) $src
		);
	}
}

Installation instructions: https://gist.github.com/westonruter/6110fbc4bef0c4b8c021a112012f7e9c

Hey Weston, I have installed this, but it seems to have broke the my logo in my header, which is now showing at a much smaller size... I think the fallback dimensions for it are off... I have deactivated and deleted the plugin, but it still looks like it is rewriting it to be WebP. Any thoughts or suggestions on how to fix this?

@mlim18 You probably have some template partials that are being cached in transients. Try deleting all transients or flushing your object cache if you have a persistent one.

I will try that out. Thanks!

Update: This is an anti-pattern which should not be used! See ampproject/amphtml#21912 (comment)

The amp-img docs that used it as an example has been removed: ampproject/amphtml#28655