I regularly deal projects that target SQL Server, but mostly develop against MySQL to avoid having to run a full Windows stack locally all the time. One of the nice things about PHP with DBAL and Migrations is that the database is pretty well abstracted from my code. Of course, this means that I don't target any of the specialist features, but for these projects, this hasn't been an issue.
To convert data from SQL Server to MySQL, I've found Intelligent Converters's MSSQL to MySQL tool to work very well for my needs. It allows me to grab the test database from the client and convert it to a MySQL dump file that I can then import.