Most improvements in PHP 5.2.x have no impact on existing code. There are a few incompatibilities and new error messages that should be considered, and code should be tested before switching PHP versions in production environments.
If the system is being upgraded from PHP 5.0.x, the manual section titled Upgrade Notes for PHP 5.1.x should also be read.
Similarly, if the system is being upgraded from PHP 4, the manual section titled Migrating from PHP 4 to PHP 5 should be read as well.