<?php
$url = 'http://www.youtube.com/watch?v=Bw4AteIEt_I&feature=feedrec_grec_index';
$nereikia = array("http://www.youtube.com/watch?v=","&feature=feedrec_grec_index");
$keiciami = array("","");
echo str_replace($nereikia, $keiciami, $url);
?>