Use the WP.com Photon image CDN without installing JetPack

wpcom-cdn.php

<?php
/**
 * Plugin Name: Photon CDN
 * Version:     1.1
 * Description: Use the WP.com Photon image CDN without installing JetPack
 * Author:      Zachary Scott
 */

namespace zacscott;

/**
 * Driver class for the plugin.
 *
 * @author Zachary Scott <[email protected]>
 */
class PhotonCDN {

    /**
     * The max number of image servers WP.com have (at time of writing it is 4)
     * So the servers as i0.wp.com, i1.wp.com, i2.wp.com, i3.wp.com
     */
    const MAXSRV = 4;

    function __construct() {
        add_action( 'wp_head', array( $this, 'dns_prefetch' ) );
        add_action( 'template_redirect', array( $this, 'start_buffering' ) );
    }

    // Adds the DNS prefetch meta fields for the WP.com servers
    function dns_prefetch() {

        for ( $srv = 0; $srv < self::MAXSRV; $srv++ ) :
            $domain = "i{$srv}.wp.com";

            ?>
            <link rel='dns-prefetch' href='//<?php echo esc_attr( $domain ) ?>' />
            <?php
        endfor;

    }

    // Start the output buffering
    function start_buffering() {
        ob_start( array( $this, 'process_output' ) );
    }

    // Processes the output buffer, replacing all matching images with URLs
    // Pointing to wp.com
    function process_output( $buffer ) {

        // Get the content directory URL minus the http://
        $content_url = content_url();
        $content_url = str_replace( 'http://', '', $content_url );
        $content_url = str_replace( 'https://', '', $content_url );

        // Replace references to images on our servers with the wp.com CDN
        return preg_replace_callback(
            '{'. $content_url .'/.+\.(jpg|jpeg|png|gif|ico|bmp)}i',
            array( $this, 'replace' ),
            $buffer
        );

    }

    // Replaces a single image URL match
    function replace( $matches ) {

        // Grab the parsed image URL
        $url = isset( $matches[0] ) ? $matches[0] : '';

        // Pick a random server
        srand( crc32( $url ) ); // Best if we always use same server for this image
        $server = rand( 0, self::MAXSRV-1 );

        // Build the wp.com URL, as return as the replacement
        return "i{$server}.wp.com/{$url}";

    }

}

// Boot
new PhotonCDN();