Basically you would do:

1. Generate a random image number, or pick from a list of prebuilt ones.
2. Save the image number to a file named with a unique session id.
3. Make the name of the form field the unique session id.
4. When submitted, check that the unique session file exists, load it, and then compare the numbers. If they match, your ok, if not, theres a problem.

Cheers,

Alex

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