How would i go about creating these in php?

The result of using $x in as a seed for the "srand" function. In this part, you may be asked to input a four digit number which then displays a new random number.

The result of encoding $x (letters only) in "rot13" using in built PHP functions.

Confused with the first question. I know how to use the srand function but only with a specific seed, and not with using $x as a seed. The second question i'm just totally lost with. Any help would be grateful. Thanks