Redirecting to AMP (reader mode) for mobile

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I’m not a big fan of AMP but I do have it active here on this blog using the official AMP plugin for WordPress, using it in “Reader” (aka “classic”) mode. That’s as far as I want to take it, but suppose (as was the case for an Autoptimize/ Critical CSS user I was helping out) you want to redirect all mobile traffic to AMP, then you could use below code snippet to do just that.

add_action('init','amp_if_mobile');
function amp_if_mobile() {
  $_request = $_SERVER['REQUEST_URI'];
	if ( strpos( $_request, '?amp' ) === false && strpos( $_request, '&amp' ) === false && ! is_admin() && wp_is_mobile() ) {
    if ( strpos( $_request, '?' ) === false ) {
      $_amp = '?amp';
    } else {
      $_amp = '&amp';
    }
    $amp_url = home_url() . $_request . $_amp;
    wp_redirect( $amp_url );
    exit;
  }
}

It might be a little rough around the edges, but it (mostly) gets the job done 😉