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Forgive if I'm wrong, but i don't think you can do this with PHP. You need to do this with your server. You can send commands to your server using PHP, but don't expect PHP to control webspace.
you need a feild (preferably in a file outsdie the webtree) then in there (i'd mask the actual value so ppl don't know what it is) you should have it set. when they have that number of entries instead of inserting you return an error