Fatal Error: Can not Redeclare...

While looking for a page on our Drupal web site, through the core Search module, a particular search term ("test") came back with a Fatal Error: Can not redeclare display_form function... This seemed to be the only word that caused this error. I finally remembered a page that had some php coding and that might have the word "test" in it. Sure enough, there was a function display_form on the page. Changing the function name fixed the problem.

So my question is: Should this be expected? Is it a requirement that any functions that we might place on a Drupal page be named such that it is not in Drupal core? Do we also need to worry about functions declared in modules installed? On the other hand, could this be an issue with my site only that might indicate a deeper problem. The search module has not been altered, but I am running the OpenSearch module as well. The page that contained the display_form function worked and displayed fine before the function name change.

Thanks for any thoughts.