To make the two interact, you do have a number of options.
Like the man said, it depends on what the meaning of "is" is. Do you need to pass a variable directly from Function A to Function B, or is it enough to stash a variable from Function A in a safe place where Function B can get hold of it later? If your user requests a second page-- or takes some action that makes the original page get redrawn-- you might even get a variable moving in the other direction, from js to php.
And then you need a backup plan in case the user doesn't have/use js, or if js and php aren't speaking to each other.