I'm getting the same problem. I just started PHP today (4/17/17) and I'm getting this bug starting on the third example using concatenation.
The following is counted as a bug and won't go through as correct.
echo "Hello," . " " . "world" . "!";
But, I used arraysolver36165 's solution of putting it all on the same line and it worked like this:
<?php echo "Hello," . " " . "world" . "!"; ?>
Probably needs a fix.