The best suggestion I have is the following:
1) Create a dump script that contains all the CREATE statements via SQL Server Enterprise Manager.
2) Manually edit the column types in the SQL CREATE statements to ones that are used in MySQL.
3) Then you can save the file as .sql and then run it in your MySQL server.
In terms of the codes between ASP and PHP...haha! That is a good one...you'll have to re-code the whole system...good luck!
Depending on the queries you've written in ASP, you could probably get away with using MySQL as your datasource, just changing your datasource references in your ASP datasource references. However, if you used SQL Server specific queries like INNER JOINS and such, they will not translate very well into MySQL. So, there will be a level of re-coding that you'll have to do anyway.