Migrating from PHP 5.0.x to PHP 5.1.x
Changes in reference handling
Migrating from PHP 5.0.x to PHP 5.1.x
PHP Manual

## Key PHP 5.1.x features

Some of the key features of PHP 5.1.x include:

- A complete rewrite of date handling code, with improved timezone support.

- Significant performance improvements compared to PHP 5.0.X.

- PDO extension is now enabled by default.

- Over 30 new functions in various extensions and built-in functionality.

- Over 400 various bug fixes.