Load Missing Media From the Live Site on Local Copies

// Production site address, without a trailing slash.
$wpide_live_url = 'https://www.example.com';

$wpide_live_media = function ( $url ) use ( $wpide_live_url ) {
    static $uploads = null;

    if ( 'production' === wp_get_environment_type() || ! is_string( $url ) || '' === $url ) {
        return $url;
    }
    if ( null === $uploads ) {
        $uploads = wp_get_upload_dir();
    }
    if ( 0 !== strpos( $url, $uploads['baseurl'] ) ) {
        return $url;
    }

    $relative = substr( $url, strlen( $uploads['baseurl'] ) );
    $local    = $uploads['basedir'] . strtok( $relative, '?' );
    if ( file_exists( $local ) ) {
        return $url;
    }

    $live_base = untrailingslashit( $wpide_live_url ) . wp_parse_url( $uploads['baseurl'], PHP_URL_PATH );
    return $live_base . $relative;
};

add_filter( 'wp_get_attachment_url', $wpide_live_media );

add_filter( 'wp_calculate_image_srcset', function ( $sources ) use ( $wpide_live_media ) {
    foreach ( (array) $sources as $width => $source ) {
        $sources[ $width ]['url'] = $wpide_live_media( $source['url'] );
    }
    return $sources;
} );

add_filter( 'the_content', function ( $content ) use ( $wpide_live_media ) {
    if ( 'production' === wp_get_environment_type() || false === strpos( $content, '/uploads/' ) ) {
        return $content;
    }
    return preg_replace_callback( '#(?<=["\'\s(])https?://[^"\'\s)]+/uploads/[^"\'\s)]+#i', function ( $match ) use ( $wpide_live_media ) {
        return $wpide_live_media( $match[0] );
    }, $content );
}, 99 );