In the first two posts of his "features new to PHP 5.4" series Slawek Lukasiewicz has posted about two things that weren't mentioned very often in most of the 5.4 hit lists - improvements to the JSON extension and the header_register_callback method.
About the JSON extension improvements:
By default, when we pass object to json_encode function, it will return JSON representation of object public properties. [...] PHP 5.4 introduces JsonSerializable interface with JsonSerialize abstract method. After implementing this method we can independently set values used in JSON representation.
Related to the header_register_callback addition
After looking at new functions introduced in PHP 5.4 we can found one called header_register_callback. Using it, we can register callback which will be called before sending output.
The stable version of PHP 5.4 has officially been released, so get out there and grab it and start using these new features now!