php widget problems

Has anyone else had a problem inserting php into their widgets and having it work??? How did you resolve it? I have tried several different plugins, but come up with the same error every time:

Fatal error: Call to undefined function search_form() in /home/xxxx/public_html/blog/wp-content/plugins/plugin name/plugin.php(100) : eval()’d code on line 1

line 1 consists of <?php

any thoughts? is there something else going wrong here? i know that my code is fine because it works on the bbpress end of things, just not on the wordpress pages. help!