Our web applications currently run against MSSQL 7.0x, Postgres 7 and MySQL. I have not been able to get MSSQL 8 (2000) and our Red Hat / Apache / PHP 4.03pl1 boxes to communicate using FreeTDS or the Sybase ct-libs. MSSQL 7.0x with FreeTDS works very well.
We use different databases depending on what is connecting other than our Linux web servers... MSSQL 7 for our Win32 CRM applications, MySQL for the lightwight stuff like bulletin boards and Postgres for documentation repositories, projects, mining and other applications that need Win32 ODBC support but aren't written specifically to MSSQL like our CRM apps are.
If I could tie everything into one SQL platform, I'm not sure that I would. These different SQL servers have their own strenghts and weeknesses.
MySQL is great for 99% read only queries. MSSQL is great for full row adds/modifications and ODBC support of Win32 applications. But MSSQL is a dog with UPDATEs operating on a single field 10000 rows down, and MySQL can't have multiple clients doing writes/updates to the same table at once which exludes it from tasks needing multiple backend data entry people.
Postgres is even tempered, reliable and supports the advanced SQL syntax akin to MSSQL that MySQL simply doesn't support. And the across the board performance is very close to our MSSQL 7 servers on less than 1/2 the hardware. (dual 800's vs single 450)
Overall, if you really want the best performing SQL server, you will have to determine the most common way it will be used and find which server excels in that area and best supports your client connection methods. You may find that true optimization will mean several different servers hidden behind a good PHP abstraction/transaction layer.
