This faq section will help you migrate from PHP 4 to PHP 5.
Migrating from PHP 4 to PHP 5
Although PHP 5 offers many new features, it's designed to be as compatible with earlier versions of PHP as possible with little functionality being broken in the process.
Be sure to read the appropriate PHP 5 migration appendix of this manual as it contains even more information on the topic of migrating to PHP 5.
Does MySQL work in PHP 5? It seemed to have disappeared.
MySQL is supported with the only change being that MySQL support is no longer enabled by default in PHP 5. This essentially means that PHP doesn't include the --with-mysql option in the configure line so that you must now manually do this when compiling PHP. Windows users will edit php.ini and enable the php_mysql.dll DLL as in PHP 4 no such DLL existed, it was simply built into your Windows PHP binaries.
Also, the MySQL client libraries are no longer bundled with PHP. More details on this topic are covered in the following FAQ and be sure to read the MySQL section for details on installing MySQL. An example configure line would be --with-mysql=/usr while Windows users will need the libmySQL.dll available to the system.
I hear PHP 5 has an entirely new OOP model, will my existing OOP code work? Where do I find information on these new OOP features?
The main change in PHP 5 is to the OOP model as PHP 5 now uses the Zend Engine 2.0. The zend.ze1_compatibility_mode directive enables compatability with the Zend Engine 1.0 (PHP 4).
So besides the new OOP model, what else has changed in PHP 5? Also, is there a PHP 5 specific version of the PHP manual?
Few other changes exist, see the migration 5 appendix for details. There won't be a PHP 5 specific version of the manual as the bulk of PHP remains the same.