Código PHP:
<?php
$cadena='[megavideo]E4KT[/megavideo]';
$patrones = '/\[megavideo\](.*?)\[\/megavideo\]/is';
$b=preg_replace($patrones, $cadena, '<object width="640" height="467"><param name="movie" value="http://www.megavideo.com/v/$1"></param><param name="allowFullScreen" value="true"></param><embed src="http://www.megavideo.com/v/$1" type="application/x-shockwave-flash" allowfullscreen="true" width="640" height="467"></embed></object>');
echo $b;
?>
gracias