ISO 8601 to seconds

iso8601.php

/**
 * Convert ISO 8601 values like PT15M33S
 * to a total value of seconds.
 * 
 * @param string $ISO8601
 */
function ISO8601ToSeconds($ISO8601)
{
    preg_match('/\d{1,2}[H]/', $ISO8601, $hours);
    preg_match('/\d{1,2}[M]/', $ISO8601, $minutes);
    preg_match('/\d{1,2}[S]/', $ISO8601, $seconds);
    
    $duration = [
        'hours'   => $hours ? $hours[0] : 0,
        'minutes' => $minutes ? $minutes[0] : 0,
        'seconds' => $seconds ? $seconds[0] : 0,
    ];

    $hours   = substr($duration['hours'], 0, -1);
    $minutes = substr($duration['minutes'], 0, -1);
    $seconds = substr($duration['seconds'], 0, -1);

    $toltalSeconds = ($hours * 60 * 60) + ($minutes * 60) + $seconds;

    return $toltalSeconds;
}

echo ISO8601ToSeconds('PT15M33S'); // Returns a value of 933

Found this gist through Google, and got warnings because you're doing math with numbers in stringformat ($hours, $minutes, $seconds). Here is a simplified version if you are using PHP 5.3 and up.
Including support for days in the input.

/**
 * Convert ISO 8601 values like P2DT15M33S
 * to a total value of seconds.
 *
 * @param string $ISO8601
 */
function ISO8601ToSeconds($ISO8601){
	$interval = new \DateInterval($ISO8601);

	return ($interval->d * 24 * 60 * 60) +
		($interval->h * 60 * 60) +
		($interval->i * 60) +
		$interval->s;
}

echo ISO8601ToSeconds('P20DT15M33S'); // Returns a value of 1728933

Good solution RuudBuger. Refrenced that code here: https://stackoverflow.com/questions/44573677/convert-youtube-api-returned-time-format-to-seconds-using-php/44573805#44573805 and credited you.