Use PHP Inside EE Entries (See related posts)

You'll need the following plugins:
Allow EE Code: http://www.pmachine.com/plugins/allow-eecode/
Allow PHP: http://loweblog.com/archive/2005/06/03/ee-allow-php-plugin/

Set text formatting for the field you want to use this on to "None" (Admin > Weblog Administration > Custom Weblog Fields).

Change your template by adding the Allow EE tags around the field:

{exp:allow_eecode}{body}{/exp:allow_eecode}

Now you can use PHP inside your entries like this:

<p>Bla bla regular entry textp>

{exp:allowphp}
echo "This will be processed as PHP.";
{/exp:allowphp}