And as to what actually happens: You don't show us the code for sendRequest( )
but assuming you are making an AJAX call, then the *actual* call to PHP will be done something like this, almost surely:
Let's say that frontEndOptionsId
is the string "zamboni".
And somewhere in your HTML you have something like
<input id="zamboni" />
and the user types in "froozle" to that text field.
So, then, you will end up doing
[ "zamboni", "froozle" ]
and I'll bet a small latte that sendRequest
will end up using the url: