What has changed in PHP 5.4.x

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:

• Upgrade Notes for PHP
  5.3.x.
• Upgrade Notes for PHP
  5.2.x.
• Upgrade Notes for PHP
  5.1.x.
• Migrating from PHP 4 to PHP
  5.