[wp-hackers] Novice question - prevent post-processing??

[Jan Erik Moström]

I'm writing my first plugin (a filter) which is also one of my 
first PHP projects. I'm a bit confused over how my code works in 
WP and I assume I've missed something. After having stripped 
away all the code that actually did something I have a filter 
that returns the following text

<div><a href="xx"> <img src="yy" /> </a> text </div>

Which shows up the same way on the wordpress page source. But if 
I change this to

<div><a href="xx"> <img src="yy" /> </a> <p>text</p> </div>

then I get

<div><a href="xx"> <img src="yy" />  </a>
<p>text</p>
</p></div>

when I check the wordpress page source. As far as I understand 
WP seem to do some processing of the code I generate ... which 
in this case generates some faulty HTML code.

Can anyone please explain what is happening and what I can do to 
prevent the processing of the code I've generated
-- 
Jan Erik Moström - www.mostrom.pp.se