I didn't imply something like [PHP]echo "Hello!";[/PHP], I mean using PHP for determining the outcome of the BBCODE. E.g. by allowing users to enter a width parameter in BBCODE, and then having PHP divide the user-entered width by an aspect ratio constant (e.g. 4/3) to determine the height of the movie clip automatically.

[youtube=400]http://www.youtube.com/v/7oXx0qwe0wc[/youtube] would thus display a hypothetical 400x300 px YouTube movie clip.

Dual parameter BBCODE would also come in handy...

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram