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.
Use pack( ) to store binary data in a string:
<?php $packed = pack('S4',1974,106,28225,32725); ?>
Use unpack( ) to extract binary data from a string:
<?php $nums = unpack('S4',$packed); ?>
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 on a little-endian machine, 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) prints:
Array (  => 1974  => 106  => 28225  => 32725 )
In unpack( ), format characters and their count can be followed by a string to be used as an array key. For example:
<?php $nums = unpack('S4num',$packed); print_r($nums); ?>
Array ( [num1] => 1974 [num2] => 106 [num3] => 28225 [num4] => 32725 )
Multiple format characters must be separated with / in unpack( ):
<?php $nums = unpack('S1a/S1b/S1c/S1d',$packed); print_r($nums); ?>
Array ( [a] => 1974 [b] => 106 [c] => 28225 [d] => 32725 )
The format characters that can be used with pack( ) and unpack( ) are listed in Table 1-2.
For a, A, h, and H, a number after the format character indicates how long the string is. For example, A25 means a 25-character space-padded string. For other format characters, a following number means how many of that type appear consecutively in a string. Use * to take the rest of the available data.
You can convert between data types with unpack( ). This example fills the array $ascii with the ASCII values of each character in $s:
<?php $s = 'platypus'; $ascii = unpack('c*',$s); print_r($ascii); ?>
Array (  => 112  => 108  => 97  => 116  => 121  => 112  => 117  => 115 )
1.16.4. See Also
Documentation on pack( ) at http://www.php.net/pack and unpack( ) at http://www.php.net/unpack .