Migrating from PHP 5.1.x to PHP 5.2.x
Migrating from PHP 5.0.x to PHP 5.1.x
Migrating from PHP 5.1.x to PHP 5.2.x
PHP Manual

## Other Enhancements

- Improved memory manager and increased default memory limit. The new memory manager allocates less memory and works faster than the previous incarnation. It allocates memory from the system in large blocks, and then manages the heap by itself. The memory_limit value in php.ini is checked, not for each emalloc() call (as before), but for actual blocks requested from the system. This means that memory_limit is far more accurate than it used to be, since the old memory manager didn't calculate all the memory overhead used by the malloc library. Thanks to this new-found accuracy memory usage may appear to have increased, although actually it has not. To accommodate this apparent increase, the default memory_limit setting was also increased - from 8 to 16 megabytes.
- Added support for constructors in interfaces to force constructor signature checks in implementations. Starting with PHP 5.2.0, interfaces can have constructors. However, if you choose to declare a constructor in an interface, each class implementing that interface MUST include a constructor with a signature matching that of the base interface constructor. By 'signature' we mean the parameter and return type definitions, including any type hints and including whether the data is passed by reference or by value.

Migrating from PHP 5.1.x to PHP 5.2.x
Migrating from PHP 5.0.x to PHP 5.1.x
Migrating from PHP 5.1.x to PHP 5.2.x
PHP Manual