Can you describe the problem in a bit more detail? Not quite sure what the problem you are having is.
Sounds like to me you have a value list of States in FileMaker, you need to make this into a seperate table in which you have the State (text) and State Number (numeric) as two fields in the table.
Use this as the basis for your dropdown (with the number field hidden) and then you can actually store the State Number in your table (as an index to the State name) and pass it along in the URL instead of the State name.
Have you ever thought of putting the MYSQL db on your relationship graph and posting the data directly within FileMaker (only if you have passwords etc)?