if you can't afford Oracle, you can use their free edition - Oracle Express Edition.
http://www.oracle.com/technology/software/products/database/xe/index.html
it's free like Microsoft SQL Server 2005 Express edition.
and there's another alternatif like Firebird/Interbase, it support stored procedure, trigger, transaction and Events.
it's not common, but Interbase/Firebird is the first choice for Delphi programmers, and you can use it in PHP too.
http://www.ibphoenix.com/main.nfs?page=ibp_beginners_php
if you want to choose between PostgreSQL and Microsoft SQL, choose Ms SQL. but if you can get Oracle, choose Oracle.
additional :
MySQL ver. 5.0 support Stored Procedure
http://dev.mysql.com/tech-resources/articles/mysql-storedprocedures.html