How to set hreflang links

I just wrote a function and popped it in the functions.php on each site (if you’re using a separate child theme for each):

add_action('wp_head', 'arwd_hreflang_head');
function arwd_hreflang_head(){
global $post;
if(is_page()) {
$url = $_SERVER['REQUEST_URI'];
?>
<link rel="alternate" href="<?= 'https://domain.co.uk'.$url ?>" hreflang="en-gb" />
<link rel="alternate" href="<?= 'https://domain.com'.$url ?>" hreflang="en" />
<?php }
}

This adds it to the header on every page on the site. My two sites are exact copies of one another so there is always a matching page on the sister site.

Thanks @arapps92

I can see your solution is simpler than ours. We use the same theme for all 3 sites, so we came up with:

function mm_add_hreflang_attribute() {
    if (!( is_singular( 'locations' ) ) ) {
        $sites = array(
            array('', 'x-default'),
            array('', 'en'),
            array('en-gb/', 'en-gb'),
            array('en-au/', 'en-au'),
        );
        
        if ( is_post_type_archive('locations') ) {
            foreach ( $sites as $site ) {
                $site_url = network_site_url();
                $page_path = 'locations/';
                $geo_url = $site[0];
                $hreflang = $site[1];
                $url = $site_url . $geo_url . $page_path;
                echo '<link rel="alternate" href="' . $url . '" hreflang="' . $hreflang . '" />'. PHP_EOL;
            }
        } elseif ( is_archive() ) {
            foreach ( $sites as $site ) {
                $site_url = network_site_url();
                $request   = $GLOBALS['wp']->request;
                $page_path = trim( preg_replace( '#^blog/topic/#', '', $request ), '/' );
                $geo_url = $site[0];
                $hreflang = $site[1];
                $url = $site_url . $geo_url . 'blog/topic/' . $page_path . '/';
                echo '<link rel="alternate" href="' . $url . '" hreflang="' . $hreflang . '" />'. PHP_EOL;
            }
        } else {
            foreach ( $sites as $site ) {
                $site_url = network_site_url();
                $page_path = substr(get_permalink(), strlen(home_url('/')));
                $geo_url = $site[0];
                $hreflang = $site[1];
                $url = $site_url . $geo_url . $page_path;
                echo '<link rel="alternate" href="' . $url . '" hreflang="' . $hreflang . '" />'. PHP_EOL;
            }
        }
    }
}
add_action( 'wp_head', 'mm_add_hreflang_attribute', 1 );