I installed PHP 5.2.11 and upon enabling the php_mssql.dll module (and the obligatory restart of the services) started receiving warnings about not being able to load the dynamic library. The operating environment is IIS ISAPI under Windows 2003 R2. Here is the list of things that were checked

1. File Exists in \PHP\ext directory and has the same timestamp as the other distribution delivered libraries.
2. Permissions are correct to the file. Other extensions are enabled and functioning with identical permissions.
3. ntwdblib.dll has been copied from a SQL Server 2005 installation (per php.net's requirements for the module)

Other modules before and after the php_mssql entry are working. I've also stripped down and remove all non-essential references with detectable improvements. I turned up logging and enabled all the error logs in the php.ini, nothing is generated by the logs.

Searching Google has turned up a slew of closed/unsolved bug reports on php.net and some miscellaneous near-matches, none of which have really addressed the problem or lead to any inspirations.