Hi Piotr,
typically I would check the variable content to make sure its both set and valid (so someone dosent try doing anything malicious) before using it in any operation. If you do that, then you will already catch errors before trying any operation.
I don't think that content can be checked in that way for http variables. Because the values of POST and GET are gathered through an eZ operator, and not via an eZ variable. And it's the operator that genrates an error, and it seems there's no way to check the exsistance of a http variable.
Maybe someone from eZ crew could give us a better answer to this.
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Nothing is impossible. Not if you can imagine it!