PHP Cookbook

1.13. Storing Binary Data in Strings

Problem

You want to parse a string that contains values encoded as a binary structure or encode values into a string. For example, you want to store numbers in their binary representation instead of as sequences of ASCII characters.

Solution

Use pack( ) to store binary data in a string:

$packed = pack('S4',1974,106,28225,32725);

Use unpack( ) to extract binary data from a string:

$nums = unpack('S4',$packed);

Discussion

The first argument to pack( ) is a format string that describes how to encode the data that’s passed in the rest of the arguments. The format string S4 tells pack( ) to produce four unsigned short 16-bit numbers in machine byte order from its input data. Given 1974, 106, 28225, and 32725 as input, this returns eight bytes: 182, 7, 106, 0, 65, 110, 213, and 127. Each two-byte pair corresponds to one of the input numbers: 7 * 256 + 182 is 1974; 0 * 256 + 106 is 106; 110 * 256 + 65 = 28225; 127 * 256 + 213 = 32725.

The first argument to unpack( ) is also a format string, and the second argument is the data to decode. Passing a format string of S4, the eight-byte sequence that pack( ) produced returns a four-element array of the original numbers:

print_r($nums);
Array
               (
                   [1] => 1974
                   [2] => 106
                   [3] => 28225
                   [4] => 32725
               )

In unpack( ), format characters and their count can be followed by a string to be used as an array key. For example:

$nums = unpack('S4num',$packed);
print_r($nums);
Array
               (
                   [num1] => 1974
                   [num2] => 106
                   [num3] => ...