PHP Examples: Handling bit-wise operations in forms.

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Handling bit-wise operations in forms.

Bit-wise operations have a variety of handy uses, one of the best ones is being able to store multiple values or flags within one variable.

For instance, say you want to store whether something can go in a number of directions. You could create 4 seperate variables for each direction - left, right, up, down and set them to 1 or 0 depending on the available directions - or a simpler method would be to have one variable with bits set to reflect the same directions.

Using the bit value method is also very useful in database applications. Instead of having to modify the table and add new columns when a new direction or object is created, you simply use an unused bit value.

The limit for this method is the size of the variable you are performing the operations on. For a 32 bit variable you can have 32 unique values - and 4294967295 possible combinations! Each of the 32 values is raised to the power of 2. 1,2,4,8,16,32,64 and so on.

An easy way to display bit values in PHP code is to use a hexidecimal format. When coding, this method also make is easier to calculate the next available number:

$a = 0x00000001 \\ 1
$b = 0x00000002 \\ 2
$c = 0x00000004 \\ 4
$d = 0x00000008 \\ 8
$e = 0x00000010 \\ 16
$f = 0x00000020 \\ 32
$g = 0x00000040 \\ 64
$h = 0x00000080 \\ 128
$i = 0x00000100 \\ 256

and so on .. the pattern becomes obvious after a while.

To set bits use the bit wise OR operator ( | ). To check whether a bit is set use the bit wise AND ( & ) operator.

The example listed below provides a simple mechanism for setting a variable with multiple bit values, and then displaying which bits are set from the same variable. Instead of directions, we are using web server capabilities.


<?php

// Obviously these values can be want ever you want,
// or they could be obtained from a database or text file
// For this example we are just using a list of web server
// capabilities

    $values = array("1"=>"PHP",            // 0x0001
                    "2"=>"Postgresql",    // 0x0002
                    "4"=>"MySQL",        // 0x0004
                    "8"=>"GD",            // 0x0008
                    "16"=>"FTP",        // 0x0010
                    "32"=>"Sockets");    // 0x0020

// When the form is posted, we scan the post variables for
// anything beginning with "bitv_".

    if(getenv("REQUEST_METHOD")=="POST")
    {
        $flag = 0;

        while (list ($key, $val) = each ($HTTP_POST_VARS)) 
        {

// We extract the actual value from the form object name and 
// perform a bitwise OR on $flag.

            if(substr($key,0,5)=="bitv_") $flag = $flag | substr($key,5);

// You could also OR the data in $val - the method shown above allows
// more flexability and also allows you pass more data in the form
//
//            if(substr($key,0,5)=="bitv_") $flag = $flag | $val;
        }
    }

// To display the values you can use code like below.
// As mentioned above we also put the bit-value associated
// with the array object in the value field of the checkbox
// object. This could be any value, as we actually extract the 
// bit-value from the checkbox name. 
// We also append the bit-value to generate a unique variable
// name for each object in the form.

    while (list($bitv,$key)=each($values))
    {
         print("<input ".
              ((($flag&$bitv)<>0)?"checked ":"").
              "name=bitv_".$bitv.
              " type=\"checkbox\" value=\"".$bitv."\">".
              $key."<br>");
    }

?>

Example

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