point search engines from dev to live WordPress site via rel="canonical"

functions.php

/*
 * Dev site: point search engines at live site
 */

// set subdomain
$subdomain = 'dev';

// test whether or not this is a dev site
if ( strpos( get_site_url(), $subdomain . '.' ) !== false ) {
    remove_action( 'wp_head', 'rel_canonical' );
    add_action( 'wp_head', 'my_rel_canonical' );
}

// rebuild canonical link
// slightly modified from the original rel_canonical function in /wp-includes/link-template.php
function my_rel_canonical() {
    global $subdomain;
    
    // original code
    if ( ! is_singular() ) {
        return;
    }
    global $wp_the_query;
    if ( ! $id = $wp_the_query->get_queried_object_id() ) {
        return;
    }

    // new code get current URL and strip dev subdomain
    $URL = str_replace( $subdomain . '.', '', get_permalink( $id ) );
    if( $URL ) {
        echo '<link rel="canonical" href="' . $URL . '" />';
        return;
    }

    // original code
    $link = get_permalink( $id );
    if ( $page = get_query_var( 'cpage' ) ) {
        $link = get_comments_pagenum_link( $page );
        echo '<link rel="canonical" href="' . $link . '" />';
    }
}