All the prev answers provide the duration in a text format:
This solution will give you the hours / minutes / seconds for you to transform to whatever format you would like:
function getDurationInSeconds($talkId){
$vidkey = $talkId;
$apikey = "xxxxx";
$dur = file_get_contents("https://www.googleapis.com/youtube/v3/videos?part=contentDetails&id=$vidkey&key=$apikey");
$VidDuration =json_decode($dur, true);
foreach ($VidDuration['items'] as $vidTime)
{
$VidDuration= $vidTime['contentDetails']['duration'];
}
preg_match_all('/(\d+)/', $VidDuration, $parts);
$hours = intval(floor($parts[0][0]/60) * 60 * 60);
$minutes = intval($parts[0][0]%60 * 60);
$seconds = intval($parts[0][1]);
$totalSec = $hours + $minutes + $seconds; // This is the example in seconds
return $totalSec;
}