A naive and incomplete, but functional approach to http2 push

tomjn_http2_push.php

<?php
/**
 * Plugin name: HTTP2 Push scripts
 * Author: Tom J Nowell
 */

function tomjn_get_dep_url( /*\WP_Dependency*/ $dep ) {
        global $wp_version;
        $relative = str_replace( site_url(), '', $dep->src );
        $ver = $dep->ver;
        if ( $ver === false ) {
                $ver = $wp_version;
        }
        return $relative.'?ver='.$ver;
}
function tomjn_push_scripts() {
        global $wp_scripts, $wp_styles;
        foreach ( $wp_scripts->queue as $handle ) {
                header("Link: <".tomjn_get_dep_url( $wp_scripts->registered[$handle] ).">; rel=preload; as=script", false);
        }
}

function tomjn_push_styles() {
        global $wp_styles;
        foreach ( $wp_styles->queue as $handle ) {
                header("Link: <".tomjn_get_dep_url( $wp_styles->registered[$handle] ).">; rel=preload; as=style", false);
        }
}
add_action( 'wp_enqueue_scripts', 'tomjn_push_scripts', PHP_INT_MAX );
add_action( 'wp_enqueue_scripts', 'tomjn_push_styles', PHP_INT_MAX );