Avoid using create_function()

To use create_function() is not a good idea. First it gets called for each rule that uses PHP code as replacement text, even if the code is not actually used; second, it doesn't allow to cache the rules into a cache table.

It would be better to pass the code to eval() when it's strictly necessary, and to keep it as text until that moment. In that way, the rule array can be saved in cache, and this would help in reducing the number of queries executed (especially when most of the time they return the same result, until a rule is not changed).

To notice that the installation code already created a cache table that is only waiting to be used. I was already writing the code for that purpose (and the code was completed), but I had to fix some problems in the code, before to think of implementing that.