Remove related videos from YouTube embeds

functions.php

// Remove related videos from youtube embeds
add_filter('embed_oembed_html', 'remove_related_videos_from_yt_oembed', 10, 3);

function remove_related_videos_from_yt_oembed($html, $url, $args = array()) {
    if(!stristr($html, 'youtube.com/embed/'))
        return $html;
 
    preg_match('/src="(.+?(?="))/', $html, $matches);
    if(count($matches) >= 2 && stristr($matches[1], 'youtube.com/embed/'))
    {
        $realUrl = $matches[1]; // the $url param is the ORIGINAL url, not the src url
 
        $newUrl = $realUrl.(stristr($realUrl, '?') ? '&' : '?').'rel=0';
        $html = str_replace($realUrl, $newUrl, $html);
    }
 
    return $html;
}