For the most part, the simple functions of database connectivity etc all you need to do is replace mysql with mssql. There are some areas which are lacking when it comes to MSSQL, but I have it working (MSSLQ 7 and IIS) with no probs (almost anyway).
Look at the PHP.net site for MSSQL in the function list, and it gives you a list of the functions. Always a good starting point (it was for me) and you can build from there.
(Note mssql_rows_affected does NOT work !!! as of PHP4.2.1 anyway)
Try this in PHP.ini
Make sure extensions=php_mssql.dll (no ; in front)
and
[MSSQL]
; Allow or prevent persistent links.
mssql.allow_persistent = On
; Maximum number of persistent links. -1 means no limit.
mssql.max_persistent = -1
; Maximum number of links (persistent+non persistent). -1 means no limit.
mssql.max_links = -1
; Minimum error severity to display.
mssql.min_error_severity = 10
; Minimum message severity to display.
mssql.min_message_severity = 10
; Compatability mode with old versions of PHP 3.0.
mssql.compatability_mode = Off
Might help things along a bit.