PHP If statement confusion

Question

I have been rooting around with PHP for a while now but I've stumbled across something that I can't make heads or tails of. It is from a post that I found on here (that I believe has been locked) and I understand most of the solution, but there's just one part that I'm confused on.

  if ( ( $number & $error_number ) == $number )
  {
    $error_description[ ] = $description;
  }

I'm not quite sure what is being checked here. Any help is appreciated.

((Full code))

<?php

$error_number = 22527; //could also use ini_get('error_reporting')
$error_description = array( );
$error_codes = array(
    E_ERROR              => "E_ERROR",
    E_WARNING            => "E_WARNING",
    E_PARSE              => "E_PARSE",
    E_NOTICE             => "E_NOTICE",
    E_CORE_ERROR         => "E_CORE_ERROR",
    E_CORE_WARNING       => "E_CORE_WARNING",
    E_COMPILE_ERROR      => "E_COMPILE_ERROR",
    E_COMPILE_WARNING    => "E_COMPILE_WARNING",
    E_USER_ERROR         => "E_USER_ERROR",
    E_USER_WARNING       => "E_USER_WARNING",
    E_USER_NOTICE        => "E_USER_NOTICE",
    E_STRICT             => "E_STRICT",
    E_RECOVERABLE_ERROR  => "E_RECOVERABLE_ERROR",
    E_DEPRECATED         => "E_DEPRECATED",
    E_USER_DEPRECATED    => "E_USER_DEPRECATED",
    E_ALL                => "E_ALL"
);
foreach( $error_codes as $number => $description )
{        
    if ( ( $number & $error_number ) == $number )
    {
        $error_description[ ] = $description;
    }
}
echo sprintf(
    "error number %d corresponds to:<br>\n%s",
    $error_number,
    implode( " | ", $error_description )
);
?>

I understand that the key's in the array are predefined PHP constants, but I'm not sure how that last if statement is working/what it's evaluating.

Thats kind of like using bit masking. If you notice the error_reporting (php.net/manual/en/function.error-reporting.php) function, you OR (|) the values; in bitwise world OR stands for "+"; — Gasim
– Gasim, Commented Jan 14, 2014 at 20:38

newfurniturey · Accepted Answer · 2014-01-14 20:40:54Z

5

The & operator is a Bitwise Operator which, when used, will return a value with the "bits" set in both variables, in this case $number and $error_number.

If the current $error_number contains the bits of $number, then it contains that error (if that makes sense?).

For example (in binary):

0001 & 1000 = 0000
0001 & 0111 = 0001
0110 & 1111 = 0110

The results show values where there is a bit set (i.e. 1) in both values being ANDed together.

Another example (with error numbers):

$error_number = E_USER_DEPRECATED | E_WARNING | E_ERROR;

if ($error_number & E_WARNING) echo 'E_WARNING'; // will output
if ($error_number & E_PARSE)   echo 'E_PARSE';   // will not output

edited Jan 14, 2014 at 20:40

answered Jan 14, 2014 at 20:35

newfurniturey

38.6k10 gold badges99 silver badges104 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Niels Keurentjes · Accepted Answer · 2014-01-14 20:35:18Z

1

& is the bitwise AND operator. Read up on how the bitwise logical operations work here.

In this case, the first number is a so-called bitfield, meaning all bits represent a different error. By performing an AND with the mask of relevant errors - if 0 results none of the requested bits was set, else one of them was.

answered Jan 14, 2014 at 20:35

Niels Keurentjes

42.1k9 gold badges103 silver badges138 bronze badges

Collectives™ on Stack Overflow

PHP If statement confusion

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related