you don't really need to know the code for this, just use the dynamic bindings and some creative Copy and Paste .
In the File Upload behavior, click the lightning bolt icon next to the eFile Name setting and select the first name binding, that will populate the name setting field with the code for the first name similar to:
<?php echo((isset($_POST["First_Name"]))?$_POST["First_Name"]:"") ?>
now Copy that (CTRL+C for Windows Or CMD+C on MAC)
Click the lightning bolt icon again and select the Last Name form element binding, this will populate the file Name setting with the binding code for the last name form element similar to:
<?php echo((isset($_POST["Last_Name"]))?$_POST["Last_Name"]:"") ?>
place your cursor at the begging of the file name input and use the Paste command (CTRL+V on Windows or CMD+V On Mac) to paste the first name binding code in front of the last name binding code, the result will be similar to:
<?php echo((isset($_POST["First_Name"]))?$_POST["First_Name"]:"") ?><?php echo((isset($_POST["Last_Name"]))?$_POST["Last_Name"]:"") ?>
Now if you also want the name of the file to be added on to the end, place your cursor at the end pf the code and add [FileName], so the full code will now be similar to:
<?php echo((isset($_POST["First_Name"]))?$_POST["First_Name"]:"") ?><?php echo((isset($_POST["Last_Name"]))?$_POST["Last_Name"]:"") ?>[FileName]
when you click finish it will generate the php code for you and set the code for the file name as:
'FileName' => "".((isset($_POST["First_Name"]))?$_POST["First_Name"]:"") ."".((isset($_POST["Last_Name"]))?$_POST["Last_Name"]:"") ."[FileName]",
