passing a variable

for a project im working on i need to pass a variable from a main page to a page im opening...basically a thumbnail will be clicked which passes a variable to a function and i need to pass that variable then on to the pop up site...any help will be appreciated..

in php you just do it like this:

link.htm?variable=something

what is the purpose of php...i realize its a server side language but what is its main function on web sites and whats it most commonly used for?

php.net