[eluser]charlie spider[/eluser]
writing
Code:
$output = $some_stuff;
does two things at once.
it declares the variable $output AND it assigns a value to it.
using .= (dot equals) is the same as writing
Code:
$output = $output + $some_stuff;
which doesn't work because the declaration of $ouput hasn't been finalized yet, so basically you can't add some_stuff to something that doesn't exist yet.