DNS Prefetching in WordPress

dns-prefetch.php

<?php
// Prefetch For Speed Up

function dns_prefetch() {

$prefetch = 'on';
    echo "\n <!-- DNS Prefetching Start --> \n";
    echo '<meta http-equiv="x-dns-prefetch-control" content="'.$prefetch.'">'."\n";
    
    if ($prefetch != 'on') {
      $dns_domains = array( 
          "//use.typekit.net",
          "//netdna.bootstrapcdn.com", 
          "//cdnjs.cloudflare.com",
          "//ajax.googleapis.com", 
          "//s0.wp.com",
          "//s.gravatar.com",
          "//stats.wordpress.com",
          "//www.google-analytics.com"
      );


      foreach ($dns_domains as $domain) {
        if (!empty($domain)) echo '<link rel="dns-prefetch" href="'.$domain.'" />'."\n";
      }
      unset($domain);
    }

    echo "<!-- DNS Prefetching end --> \n";

}

add_action( 'wp_head', 'dns_prefetch', 0 );

?>

any reason you're not filtering wp_resource_hints() ?