have about 16 mil file n ders no way i can change every thing
One option is to get your web host to configure the server to treat .php3 files as PHP5 scripts. But, of course it should be possible to change everything. For example, you could write a PHP script to change all the filenames and even (very carefully) perform search and replace within the files.
That said, if you really have PHP3 scripts, you are probably out of luck and should get a PHP5.2 compatible substitute for whatever script you are using. If you wrote the scripts yourself, I do wonder what on earth persuaded you to use the .php3 file extension :p