Question about the "Not" Operator

[hostfreak]

I see it used often, e.g:

[code]
if (!isset($variable) { ...
[/code]

But, I am not sure of a practical use for it? I have been wondering for awhile now, it is really starting to bug me. Any answers are appreciated, thanks.

[btherl]

If you did

[code]if (isset($variable))[/code]

Then you would be checking if $variable is set.

[code]if (!isset($variable))[/code]

checks if $variable is NOT set.  Basically, it gives you the opposite result to what you would normally get.  Another example:

[code]if ($var > 5) { print "$var is greater than 5\n";}
if (!($var > 5)) { print "$var is NOT greater than 5\n";}[/code]

The second is the exact opposite of the first.

[kenrbnsn]

Most of the time I use this when I use the same script to process a form and to display it. In the form my submit button is usually named "thesubmitbutton" or something like that. This comes to my script in the $_POST array and is only defined when then form is submitted, so I can do something like:
[code]<?php
if (isset($_POST['thesubmitbutton'])) {
//
// process form
//
} else {
//
// display form
//
}?>[/code]
Another example would be storing a session variable, but only if it's not already set.
[code]<?php
session_start();
if (!isset($_SESSION['my_var'])) $_SESSION['my_var'] = 'some value here';
?>[/code]

Ken

[hostfreak]

Ah, makes perfect sense now. Thanks guys.