在FireFox webm中不支持但在Chrome中支持相同的webm

Here is my code:

<?php

/**
 * @author Arpit
 * @copyright 2012
 */

$crxml=file_get_contents('latest.xml');
$rss=new SimpleXMLElement($crxml);
$playlistitem=1;
foreach($rss->channel->item as $post)
{

    if($playlistitem<=8)
    {    
    echo '<ul>';
    echo '<li class="xdesc">'.$post->description.'</li>';

    $dc=$post->children('http://search.yahoo.com/mrss/');
    echo '<li class="xthumb">'.$dc->thumbnail->attributes()->url.'</li>';
    echo '<li class="xpreview">'.$dc->thumbnail->attributes()->url.'</li>';

    echo '<li class="xsources_mp4">'.$dc->content->attributes()->url.'</li>';

   $dc->content->attributes()->url=preg_replace('/.mp4/','.webm',$dc->content->attributes()->url);

    echo '<li class="xsources_webm">'.$dc->content->attributes()->url.'</li>';
    echo '</ul>';
    $playlistitem++;
    }

}


?>

When I change the extension to webm in both xsources_mp4 and xsource_webm then it again work in chrome but in Firefox it show undefined when i Firebug it and the video fail yo play.

Different browsers use different formats for playing content, some event support more than one. Check this wiki which browser supports what.