JSLint: unexpected '?' Follow

Not sure if this is a bug or some mistake I don't get.
PhpStomr 5.0.3 is showing me this message on each first line of any php script that obviously start with "<?php":

     Name:
          grand_total_helper.php

     Location:
          file grand_total_helper.php

     Problem      synopsis:
          JSLint:      Unexpected '?'. (at line 1)

Then it also shows something like that:
     Name:
          grand_total_helper.php

     Location:
          file grand_total_helper.php

     Problem      synopsis:
          JSLint:      Stopping. (2% scanned). (at line 1)



Actually this happens also with an xml file I've included in the same project and that (obviously) start with '<?xml version="1.0" encoding="UTF-8"?>'.

Any guess of what is causing this and how to get rid of it?