What is !$this

[Mutley]

I tried searching but it never picks up punctuation, anyway.

If I see a variable with an explanation what does it mean? Where can I find a list of definitions for this type of thing
if([b]![/b]$this);

[taith]

$this=true;
if($this) echo true; #$this==true
if(!$this) echo false; #$this==false

[Hypnos]

It also works similar to isset, only it will return a FALSE if the string is set but empty.

[code=php:0]
$a = "A";
$b = "";

if($a) echo "\$a is not empty";
else echo "\$a is empty";

if($b) echo "\$b is not empty";
else echo "\$b is empty";
[/code]

This code would echo:
$a is not empty
$b is empty

If you used if(isset()) instead, it would say that both are not empty.

EDIT:
And of course "!" just says "if it is not".
http://www.w3schools.com/php/php_operators.asp