Although most existing PHP 5 code should work without changes, you should pay attention to the following backward incompatible changes:
getrusage() returns NULL when passed incompatible arguments as of PHP 5.2.1.
ZipArchive::setCommentName() returns TRUE on success as of PHP 5.2.1.
ZipArchive::setCommentIndex() returns TRUE on success as of PHP 5.2.1.
SplFileObject::getFilename() returns the filename, not relative/path/to/file, as of PHP 5.2.1.
Changed priority of PHPRC environment variable on Win32
The PHPRC environment variable now takes priority over the path stored in the Windows registry.
CLI SAPI no longer checks cwd for php.ini or the php-cli.ini file
In PHP 5.1.x an undocumented feature was added that made the CLI binary check the current working directory for a PHP configuration file, potentially leading to unpredictable behavior if an unexpected configuration file were read. This functionality was removed in 5.2.0, and PHP will no longer search CWD for the presence of php.ini or php-cli.ini files. See also the command line section of the manual.
Added a warning when performing modulus 0 operations
In earlier versions of PHP, performing integer % 0 did not emit any warning messages, instead returning an unexpected return value of FALSE. As of PHP 5.2.0, this operation will emit an E_WARNING, as is the case in all other instances where division by zero is performed.
Changed __toString() to be called wherever applicable.
The magic method __toString() will now be called in a string context, that is, anywhere an object is used as a string.
The fallback of returning a string that contains the object identifier was dropped in PHP 5.2.0. It became problematic because an object identifier cannot be considered unique. This change will mean that your application is flawed if you have relied on the object identifier as a return value. An attempt to use that value as a string will now result in a catchable fatal error.
Even with __toString(), objects cannot be used as array indices or keys. We may add built-in hash support for this at a later date, but as of PHP 5.2.x you will need to either provide your own hashing or use the new SPL function spl_object_hash().
Exceptions can not be thrown from __toString() methods.
Dropped abstract static class functions.
Due to an oversight, PHP 5.0.x and 5.1.x allowed abstract static functions in classes. As of PHP 5.2.x, only interfaces can have them.
Added RFC2397 (data: stream) support.
The introduction of the 'data' URL scheme has the potential to lead to a change of behavior under Windows. If you are working with a NTFS file system and making use of meta streams in your application, and if you just happen to be using a file with the name 'data:' that is accessed without any path information - it won't work any more. The fix is to use the 'file:' protocol when accessing it.
See also RFC 2397