Given that Linux/Unix users can add so much functionality to PHP, just through various build options, it is likely that you will upgrade or modify PHP more often than Apache or MySQL. Regardless of whether you are upgrading to a new minor version or simply adding new functionality (or removing some you no longer need), the process for modifying an existing version is exactly the same as installing it in the first place: configure, make, make install. The make install step will place the PHP module in the appropriate place in the Apache directory tree. Once your new module is in place, restart Apachethe new version of PHP should be in use.
If you are upgrading to a new minor version of PHP, when you extract the distribution archive you will have a completely distinct directory tree, based on the version number. Perform the configure, make, make install steps within this new directory structure and a new PHP module will be built, independent of the other.
Windows users have a different set of tasks to perform; adding new functionality to an existing module requires only that the module be activated through changes to php.ini and the server restarted. Upgrading to a new minor version requires you to download a new distribution file. The contents of this file are then extracted into a directory named for the version it represents. You must then follow the steps required for installation, regarding the placement of php.ini and php4ts.dll files, as each version produces a different file. Finally, change any PHP-related pathnames in the Apache httpd.conf file, and restart the serverthe new version of PHP should be in use.