On the main PHP site there's a note about the latest release of the language - PHP 5.1.6.

The PHP development team would like to announce the immediate availability of PHP 5.1.6. This release contains a fix for memory_limit restriction on 64 bit systems that was not included in PHP 5.1.5.

They also corrected this bug, an issue with the "php://stdin" (and the like) functionality that caused crashes on Windows systems.

You can grab this latest update from the Downloads page in both the source and Windows binaries distributions.