PHP Strings Storing Binary Data in Strings

PHP Strings

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 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
               (
                        [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);

This prints:

               Array
               (
                       [num1] => 1974
                       [num2] => 106
                       [num3] => 28225
                       [num4] => 32725
               )

Multiple format characters must be separated with / in unpack():

               $nums = unpack('S1a/S1b/S1c/S1d',$packed);
               print_r($nums);

This prints:

               Array
               (
                       [a] => 1974
                       [b] => 106
                       [c] => 28225
                       [d] => 32725
               )

Format characters for pack( ) and unpack( )

Format character  Data type
a                              NUL-padded string
A                             Space-padded string
h                              Hex string, low nibble first
H                             Hex string, high nibble first
c                              signed char
C                             unsigned char
s                               signed short (16 bit, machine byte order)
S                              unsigned short (16 bit, machine byte order)
n                              unsigned short (16 bit, big endian byte order)
v                              unsigned short (16 bit, little endian byte order)
i                               signed int (machine-dependent size and byte order)
I                               unsigned int (machine-dependent size and byte order)
l                               signed long (32 bit, machine byte order)
L                              unsigned long (32 bit, machine byte order)
N                             unsigned long (32 bit, big endian byte order)
V                             unsigned long (32 bit, little endian byte order)
f                               float (machine-dependent size and representation)
d                              double (machine-dependent size and representation)
x                              NUL byte
X                             Back up one byte
@                            NUL-fill to absolute position

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 $as cii with the ASCII values of each character in $s:

                $s = 'platypus';
                $ascii = unpack('c*',$s);
                print_r($ascii);

This prints:

                 Array
                 (
                        [1] => 112
                        [2] => 108
                        [3] => 97
                        [4] => 116
                        [5] => 121
                        [6] => 112
                        [7] => 117
                        [8] => 115
                 )