Southern writes:  

Table of Contents

What has changed in PHP 5.3.x
Backward Incompatible Changes
New features
Changes made to Windows support
Changes in SAPI modules
Deprecated features in PHP 5.3.x
Undeprecated features in PHP 5.3.x
New Parameters
New Functions
New stream wrappers
New stream filters
New Class Constants
New Methods
New Extensions
Removed Extensions
Other changes to extensions
New Classes
New Global Constants
Changes to INI file handling
Other changes

See also the migration guides for PHP versions 5.0.x, 5.1.x and 5.2.x.
more: us2.php.net