The key features of PHP 5.4.0 include:

• New language syntax including Traits, shortened array syntax and more
• Improved performance and reduced memory consumption
• Support for multibyte languages now available in all builds of PHP at the flip of a runtime switch
• Built-in webserver in CLI mode to simplify development workflows and testing
• Cleaner code base thanks to the removal of multiple deprecated language features
• Many more improvements and fixes

Changes that affect compatibility:

• Register globals, magic quotes and safe mode were removed
• The break/continue $var syntax was removed
• The ini option allow_call_time_pass_reference was removed
• The PHP default_charset is now “UTF-8” within the distributed php.ini files, but still defaults to “”

Extensions moved to PECL:

• ext/sqlite (ext/sqlite3 and ext/pdo_sqlite are not affected)

For users upgrading from PHP 5.3 there is a migration guide available here, detailing the changes between PHP 5.3 and PHP 5.4.0.