Código PHP:
<pre><?php
$video = <<<HTML
<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/WwqqHpXFkGk"/>
<param name="allowFullScreen" value="true"/>
<param name="allowsc#iptaccess" value="always"/>
<embed src="http://www.youtube.com/v/WwqqHpXFkGk&hl=es&fs=1"
type="application/x-shockwave-flash" allowsc#iptaccess="always" allowfullscreen="true" width="425" height="344"/>
</object>
HTML;
$video = preg_replace('/width="(\d+)"/', 'width="400"', $video);
$video = preg_replace('/height="(\d+)"/', 'height="260"', $video);
echo $video;
?></pre>
