I was wondering if the following is possible... 1) A textbox is on the screen and someone enters text like "Sample Text". 2) The text "Sample Text" gets encoded safely so it can appear in a querystring and not be tampered with. 3) Text can be decoded safely from the querystring. If the text is tampered with in any way, the decoding process will know this and will raise a flag / display an error etc. How can I do this in PHP?