What do you think? Do you believe the open source community designed php4 to be incompatible with php3, so that you couldn't upgrade? Usually, the way it works is that new versions expand the language, rather than narrow it.
I have yet to read a stupid question in this forum. Although I have read some stupid answers (some of which have been penned by me).
When you install PHP, there should be two php.ini files included with the install. One will be called php.ini-dist and the other is called php.ini-optimized. If you copy and rename the former to be php.ini you will have no backwards compatability problems.