Why can I use PHP in some of my Javascript but not in specific places?

I am fundamentally confused about something in my code.

If I code this on one of my .php pages:

$userEmail = $_SESSION["inputEmail"];

and then use it in an AJAX call inside one of my Javascript functions:

function firstFunction() {
    $.ajax({
        url: someUrl.php?inputEmail='
        sync: true,
        success: function(result) {
          $("#someDiv").html(result)

         }
      })
}

I end up getting the result I need. I am confused, however, because if in this function, I did something like this:

function secondFunction() {
    if ( == "test@test.com") {
        //do some stuff

    }
}

It does not work. I have read answers to this exact scenario right above, but I don't understand why the first line of code I provided in this example works but this second one does not.

Initially, I expected the Javascript to be able to read $userEmail in the function above, but after viewing the page source, I see it is evaluation nothing. It looks like:

if(  == "test@test.com)

Like I said, I've looked this part up. But now I'm even more confused. Why can I do the AJAX call under firstFunction() but not secondFunction()