1. that depends on how you installed it.
- if you’re using WAMP or XAMPP, a reinstall of these programmes should suffice (note that they have to state in the description what PHP version they use).
- if you installed via php.net’s provided installer, uninstall it and install the new version.
- if you compiled from scratch, refresh the sources and recompile.
2. forgetting to save your previous php.ini settings
3. occasionally, default behaviour changes. e.g. register_globals is deprecated in PHP 5.3, XML handling changed radically from PHP 4 to PHP 5. nevertheless, all such changes are mentioned in the manual.
please post your code wrapped in [CODE] [/CODE] tags