Just started doing that myself. The following main functions are completely analogous to their mssql counterparts
mysql_connect (e.g. use mssql_connect the same way)
mysql_select_db
mysql_query
mysql_num_rows
mysql_fetch_array
However, mysql_error() doesn't have a corresponding mssql_error() function, the error reporting is radically different.
Remember that not all the SQL queries you're so used to with mysql will work in mssql, so you may need ot change your functions as well.
Sam