How expensive are function_exists() calls?

I am trying to figure out a strategy for multiple output formats on a site, and it seems like I can have functions defined by default, but have them defined -after- I've included the targetted format first. However that would require file1.php: function foo() {} file2.php if(!function_exists('foo')) { function foo() {} } Is this a very expensive thing to do? There would be a good number of functions that would be leveraging this. Is it a no-no? or is it pretty cheap? Thanks - mike