Most improvements in PHP 5.5.x have no impact on existing code. There are a few incompatibilities and new features that should be considered, and code should be tested before switching PHP versions in production environments.
For systems being upgraded from an older version of PHP, the relevant documentation is available at:
Most improvements in PHP 5.4.x have no impact on existing code. There are a few incompatibilities and new features that should be considered, and code should be tested before switching PHP versions in production environments.
For systems being upgraded from an older version of PHP, the relevant documentation is available at:
Most improvements in PHP 5.3.x have no impact on existing code. There are a few incompatibilities and new features that should be considered, and code should be tested before switching PHP versions in production environments.
For systems being upgraded from an older version of PHP, the relevant documentation is available at:
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.
Some of the key features of PHP 5.1.x include:
A complete rewrite of date handling code, with improved timezone support.
Significant performance improvements compared to PHP 5.0.X.
PDO extension is now enabled by default.
Over 30 new functions in various extensions and built-in functionality.
Over 400 various bug fixes.
PHP 5 and the integrated Zend Engine 2 have greatly improved PHP's performance and capabilities, but great care has been taken to break as little existing code as possible. So migrating your code from PHP 4 to 5 should be very easy. Most existing PHP 4 code should be ready to run without changes, but you should still know about the few differences and take care to test your code before switching versions in production environments.