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Date: Thu, 14 Aug 2003 12:51:39 -0700
In-reply-to: <200308141559.05609.ilia@prohost.org>
To: ilia@prohost.org
Cc: Robert Cummings <robert@wocmud.org>, walt boring <waboring@3gstech.com>,
 Timm Friebe <thekid@thekid.de>, internals@lists.php.net
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Subject: Re: [PHP-DEV] Re: variable_exists() patch
From: torben@php.net (Lars Torben Wilson)

On Thu, 2003-08-14 at 12:59, Ilia Alshanetsky wrote:
> On August 14, 2003 03:33 pm, Robert Cummings wrote:
> > Actually isset() doesn't behave as it should:
> >
> >     $foo = null;
> >     echo isset( $foo );
> 
> Not quite. To understand the nature of NULL you must consider the following. 
> Suppose you have a variable $foo you wish to 'destroy' you can do so by doing 
> unset($foo) or $foo = NULL;. In both cases the value of $foo will be 
> destroyed, however the variable will remain, it's value will become NULL. 
> Therefor isset() behaviour, which works by seeing if a variable exists and 
> making sure that its value is not null, is correct and now flawed as you 
> claim. This is true for other languages as well such as C, when a pointer's 
> value is null that pointer is 'no set'.
> 
> Ilia

unset($foo) is not the same as $foo = NULL, which is one way this
is useful:

<?php
echo "\$foo = null\n\n";
$foo = null;
echo "variable_exists(\$foo): " . (variable_exists($foo) ? 'yes' : 'no')
. " (should be yes)\n";
echo "isset(\$foo): " . (isset($foo) ? 'yes' : 'no') . " (should be
yes)\n";
echo "unset(\$foo);\n";
unset($foo);
echo "variable_exists(\$foo): " . (variable_exists($foo) ? 'yes' : 'no')
. " (should be no)\n";
echo "isset(\$bar): " . (isset($bar) ? 'yes' : 'no') . " (should be
no)\n";
?>

OUTPUT:
$foo = null
variable_exists($foo): yes (should be yes)
isset($foo): no (should be yes)
unset($foo);
variable_exists($foo): no (should be no)
isset($bar): no (should be no)


-- 
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