I build mssql.dll from last source code of PHP v.4.3 betta under Microsoft Visual C++ 5.0. It's compiled and build OK.
After this I copied it into my PHP directory and used

dl('mssql.dll');

in my php code. (Error was written to syslog of NT)
After code execution I see:

The description for Event ID ( 2000 ) in Source ( c-client ) could not be found. It contains the following insertion string(s): c-client[296], PHP Warning:  Invalid library (maybe not a PHP library) 'mssql.dll'  in Unknown on line 0.

Thank you.