useful link: http://drupal.org/handbook/customization/php-snippets
to get php in blocks, enable the Php Filter in the modules section.
I've managed to break a site (so that nothing renders because of a php error in a block) and fix it by posting a page that I already had open with that block being edited.
I haven't had to work out how to perform such a fix without that lucky open page (yet).
