Finnally i use the str_replace and displays both videos.Here's is my code , maybe will help someone in the future
<?php
$html = "";
$url = "http://123.gr/index.php?format=feed&type=rss";
$xml = file_get_contents($url);
$x = new SimpleXmlElement($xml);
foreach ( $x->channel->item as $entry ){
$part_to_replace = array("http://123.gr///www.youtube.com/");
$replaced = array("http://www.youtube.com/");
$entry->description = str_replace( $part_to_replace, $replaced, $entry->description );
echo $entry->description;
echo $entry->title;
?>