Perfetto.
Ho imparato ben due cose, grazie.

Per chi volesse, ecco la index.php. Mi serve per la costruzione di una galleria video.

codice:
<?php
print "<form action=\"index.php\" method=\"get\">
<input type=\"text\" name=\"link\" />
<input type=\"submit\" value=\"Cerca\" />
</form>\n";
if (!empty($_GET["link"])) {
$link=$_GET["link"];
preg_match("#(?:http://)?(?:www\.)?youtube\.(?:com|it)/(?:watch\?v=|v/)(.{11})#i", $link, $parte);
header("location: index.php?yt=".$parte[1]);
}
if (!empty($_GET["yt"]))
print "<object width=\"480\" height=\"385\"><param name=\"movie\" value=\"http://www.youtube.com/v/".$_GET["yt"]."?fs=1&amp;hl=it_IT\"></param><param name=\"allowFullScreen\" value=\"true\"></param><param name=\"allowscriptaccess\" value=\"always\"></param><embed src=\"http://www.youtube.com/v/".$_GET["yt"]."?fs=1&amp;hl=it_IT\" type=\"application/x-shockwave-flash\" allowscriptaccess=\"always\" allowfullscreen=\"true\" width=\"480\" height=\"385\"></embed></object>";
?>