Cal Evans came up against a problem earlier in his development work - issues when he was trying to get his old include path.
What you SHOULD get back is a string describing your old include path. If you get false, you have hit the problem I hit last night. I went so far as to recompile 2 difference versions of Apache thinking (almost correctly) that it was something that had changed in my Apache config files.
If you're one of the "falsers" out there, check out the rest of his post of his solution around the problem. His problem stemmed from his trying to reassign a php_admin_value (instead of php_value where the script could override it) setting, specifically the one for the include_path.
He also points out that Zend Framework users might keep an eye out for this problem too as the Zend_Loader package changes the include path at times.