To make this work you need to go to Google Developers Console, create a project, enable the YouTube Data API v3, and get a key in authentication key in credentials. This is pretty much the difficult part.
Now the easy part, so easy and keystroke economic to do this in PHP… just a couple of lines, and it’s done:
define("API_KEY", ""); // Fill in your Google API Key
function getYoutubeDurationV3($id) {
$json = json_decode(
file_get_contents('https://www.googleapis.com/youtube/v3/videos'.
'?part=contentDetails&d='.$id.'&key='.API_KEY)
);
$start = new DateTime('@0');
$youtube = new DateTime('@0');
$youtube->add(new DateInterval($json->items[0]->contentDetails->duration));
return $youtube->getTimestamp() - $start->getTimestamp();
}