PHP A to ZCE: Bitwise Operations

By Quentin Zervaas, 3 November 2010 PHP ZCE Download PDF

This article is part of the series “PHP A to Zend Certified Engineer”. In PHP A to ZCE, I will take you through 26 different yet equally important topics that will help you become a Zend Certified Engineer. Even if you're not interested in sitting the ZCE-PHP exam, these topics will elevate your understanding of PHP to a whole new level and allow you to become the guru in your company. Read more about PHP A to Zend Certified Engineer...

Bitwise operations allow evaluation and manipulation of specific bits within an integer. In this article I will show each of the bitwise operations that are available and how to practically use them within PHP.

About Bitwise Operations

Primarily used as a series of on/off "switches".
Each switch corresponds to a single bit in an integer
Bitwise operators allow you to turn on or turn off various switches
Easiest to think of terms of binary numbers and operate with hex values
Bits are read from right-to-left

As an example, the following shows breaking down the number 13 into binary digits. Think of this number as a series of switches, the first, third and fourth "switches" are enabled (reading from the right).

Listing 9 listing-9.txt

Position: 128 64 32 16  8  4  2  1
----------------------------------
Number:     0  0  0  0  1  1  0  1

Available Operators

There are six bitwise operations available. All operations use two values (that is, they have a left and a right operand) except for "not", which is a unary operator (has a single operand only).
The operations and their result are as follows:

And ($c = $a & $b;): Bits that are set in both $a and $b are set in $c.
Or ($c = $a | $b): Bits that are set in either $a and $b are set in $c.
Exclusive Or (Xor) ($c = $a ^ $b): Bits that are set in one and not the other are set in $c.
Not ($c = ~$a): Bits that are set in $a are not set in $c. Bits that are not set in $a are set in $c.
Shift Left ($c = $a << $b): $b indicates the number of positions the bits in $a are to move left. Each position move in effect is the same as multiplying by 2.
Shift Right ($c = $a >> $b): $b indicates the number of positions the bits in $a are to move right. Each position move in effect is the same as dividing by 2 (the remainder is ignore since we're operating on integers).

PHP also supports the following syntax:

$a = $a & $b; can be shortened to $a &= $b;
$a = $a | $b; can be shortened to $a |= $b;
$a = $a ^ $b; can be shortened to $a ^= $b;

Hexadecimal Numbers

Every four bits can be represented by a single hex digit
Hex is 0-9, A=10, B=11, C=12, D=13, E=14, F=15
The number 13 can be represented by D in hex
All hex numbers are preceded by 0x, meaning 13 is actually 0xD.
All switches on: 0xF - all switches off: 0x0
You can output hex numbers using the %X switch in sprintf()

The And Operation

Bits that are set in both $a and $b are set in $c.

Listing 1 listing-1.php

<?php
    $a = 0xD; // 13 in decimal, 1101 in binary
    $b = 0x7; // 7 in decimal 0111 in binary
 
    $c = $a & $b;
 
    /*
          1101  (13, 0xD)
        & 0111  (7, 0x7)
          ----
          0101  (5, 0x5)
    */
 
    echo $c; // outputs 5
?>

The Or Operation

Bits that are set in either $a and $b are set in $c.

Listing 2 listing-2.php

<?php
    $a = 0xD; // 13 in decimal, 1101 in binary
    $b = 0x7; // 7 in decimal 0111 in binary
 
    $c = $a | $b;
 
    /*
          1101  (13, 0xD)
        | 0111  (7, 0x7)
          ----
          1111  (15, 0xF)
    */
 
    echo $c; // outputs 15
?>

The Xor (Exclusive Or) Operation

Bits that are set in one and not the other are set in $c.

Listing 3 listing-3.php

<?php
    $a = 0xD; // 13 in decimal, 1101 in binary
    $b = 0x7; // 7 in decimal 0111 in binary
 
    $c = $a | $b;
 
    /*
          1101  (13, 0xD)
        ^ 0111  (7, 0x7)
          ----
          1010  (10, 0xA)
    */
 
    echo $c; // outputs 10
?>

The Not Operation

Bits that are set in $a are not set in $c. Bits that are not set in $a are set in $c.

Listing 4 listing-4.php

<?php
    $a = 0xD; // 13 in decimal, 1101 in binary
 
    $c = ~$a
 
    /*
        ~ 1101  (13, 0xD)
          ----
          0010  (2, 0x2)
    */
 
    echo $c; // outputs 2
?>

The Shift Left Operation

$b indicates the number of positions the bits in $a are to move left. Each position move in effect is the same as multiplying by 2.

Listing 5 listing-5.php

<?php
    $a = 0xD; // 13 in decimal, 1101 in binary
    $b = 2;
 
    $c = $a << $b;
 
    /*
          000001101  (13, 0xD)
          ---------
             << 2
          ---------
          000110100  (52, 0x34)
    */
 
    echo $c; // outputs 52
    echo $a * 2 * 2; // outputs 52
?>

The Shift Right Operation

$b indicates the number of positions the bits in $a are to move right. Each position move in effect is the same as dividing by 2 (the remainder is ignore since we're operating on integers).

Listing 6 listing-6.php

<?php
    $a = 0xD; // 13 in decimal, 1101 in binary
    $b = 1;
 
    $c = $a >> $b;
 
    /*
          000001101  (13, 0xD)
          ---------
             >> 1
          ---------
          000000110  (6, 0x6)
    */
 
    echo $c; // outputs 6
    echo floor($a / 2); // outputs 6
    // floor is used because the output must be an int
?>

Practical Example 1: PHP Error Reporting

PHP's error reporting system is an example of using bitwise operations and switches
You can change the reporting level by turning switches on or off and passing the result to error_reporting()

Listing 10 listing-10.txt

E_ERROR   = 1 = 0001 = 0x1
E_WARNING = 2 = 0010 = 0x2
E_PARSE   = 4 = 0100 = 0x4
E_NOTICE  = 8 = 1000 = 0x8

To enable errors and warnings, use E_ERROR | E_WARNING

Listing 11 listing-11.txt

E_ERROR   = 0001
E_WARNING = 0010
            ----
            0011

You can check if a particular level is enabled using "and"
If that switch is not enabled the result will be zero (0x0)

Listing 7 listing-7.php

<?php
    error_reporting(E_ERROR | E_WARNING);
 
    if (error_reporting() & E_WARNING) {
        // E_WARNING is enabled
    }
 
    /*
        error_reporting(): 0011
        E_WARNING:         0010
                           ----
                         & 0010 - Therefore E_WARNING is enabled!
     */
?>

Practical Example 2: IP Addresses and Subnet Masks

You can use bitwise operations to see if an IP address is within a given subnet
You need the subnet and the subnet mask to determine this
You also need to convert the IP address to an integer representation so bitwise operations will work: ip2long() and long2ip() help with this.

Listing 8 listing-8.php

<?php
    $ips = array(
        '192.168.0.25',
        '10.0.0.1'
    );
 
    $subnet     = ip2long('192.168.0.0');
    $subnetMask = ip2long('255.255.255.0');
 
    foreach ($ips as $ip) {
        $longIp = ip2long($ip);
 
        if (($longIp & $subnetMask) == $subnet) {
            echo sprintf("%s is in %s subnet\n", $ip, long2ip($subnet));
        }
        else {
            echo sprintf("%s is not in %s subnet\n", $ip, long2ip($subnet));
        }
    }
 
    /*
        Output:
        192.168.0.25 is in 192.168.0.0 subnet
        10.0.0.1 is not in 192.168.0.0 subnet
    */
?>

Note: Be sure to use brackets appropriately. If you don't use brackets in the above if statement, the == comparison is performed before the bitwise operation.

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