Make a random variables

Hello,

I'm trying to make strings random in url. For example:

Url is: http://google.com/?index.php?$variable1=15&variable2=20&variable=30and go on.

I need to make them random like some times $variable1=15&variable2=20&variable=30, sometimes $variable1=15, somtimes $variable1=15&variable2=20

Is there any ideas? I have tried with random() but is not that accurate what I needed. Probably is with array or something else.

Thank you.