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Bug #45518 NULL matching zero, empty array, false
Submitted: 2008-07-15 12:19 UTC Modified: 2008-07-15 12:23 UTC
From: stephen dot cuppett at sas dot com Assigned:
Status: Not a bug Package: Variables related
PHP Version: 5.2.6 OS: Windows XP/2003
Private report: No CVE-ID: None
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From: stephen dot cuppett at sas dot com
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 [2008-07-15 12:19 UTC] stephen dot cuppett at sas dot com
I noticed a behavior with PHP 5.2.x that I found disturbing.  PHP is matching variables initialized to zero, an empty array and false with NULL.  From the documentation here:

The only thing that should match NULL are ones that meet these criteria:

A variable is considered to be null if: 

 - it has been assigned the constant NULL. 
 - it has not been set to any value yet. 
 - it has been unset(). 

The function empty() should return true in the above cases (and does), but a comparison operator of == should not.  FYI, is_null() returns false and the === operator works as expected too.

Reproduce code:
  $empty_array = array();
  if ($empty_array == NULL)
    echo "Empty array, same as NULL\n";
    echo "Empty array, diff from NULL\n";

  $zero = 0;
  if ($zero == NULL)
    echo "Zero, same as NULL\n";
    echo "Zero, different from NULL\n";

Expected result:
Empty array, diff from NULL
Zero, different from NULL

Actual result:
Empty array, same as NULL
Zero, same as NULL


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 [2008-07-15 12:23 UTC] stephen dot cuppett at sas dot com
NM, I see the loose comparisons table in the appendix.
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