Some of the key features of PHP 5.1 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.
Bundled libraries, PCRE and SQLite upgraded to latest versions.
Over 400 various bug fixes.
PEAR upgraded to version 1.4.5