- str_replace: documentation ( source)
- preg_replace: documentation ( source)
<?php
$video = "https://www.youtube.com/watch?v=9g30pPikveg&feature=youtu.be";
$replace = str_replace('&feature=', '', $video);
echo $replace;
$pattern = '#https?://(?:www\.)?youtube\.com/watch\?v=#';
$replacement = 'http://www.youtube.com/embed/';
$final = preg_replace($pattern, $replacement, $video);
//echo $final;