Cannot use isset() on the result of an expression (you can use "null !== expression" instead)

[matjazpotocnik]

Hi!

I was testing PW installation on PHP 5.6 and I got:

Cannot use isset() on the result of an expression (you can use "null !== expression" instead)

in line 221:

if (!isset(self::fileTypes[$ext])) return false;

Fix:

$a = self::fileTypes[$ext];
if (!isset($a)) return false;