$iframe_string = '<iframe width="100%" height="315" src="https://www.youtube.com/embed/-XC6r9SIQfo" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>';


preg_match('/src="([^"]+)"/', $iframe_string, $match);

/*Output all the matches by the expression*/
print_r($match);

/*Get the index of the array*/
$url = $match[1];
echo $url;