Inputing multiple form options in SET

Hello.

I have an HTML form with multiple options that I would like to input into a MySQL database as a SET. When the form is submitted, the data is sent as Variable=1&Variable=2&Variable=3, etc, while MySQL appears to want it submitted as Variable=1,2,3. Is there a way to make this work?

Thank you!

$newvariable="$variable1,$variable2,$variable3";

If you are unsure how many will be in the original set you'd have to concatenate dynamically.