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1.14. Storing Binary Data in Strings

1.14.3. 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] => 28225
    [num4] => 32725
)

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

$nums = unpack('S1a/S1b/S1c/S1d',$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.

Table 1-2. 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 $ascii with the ASCII values of each character in $s:

$s = 'platypus';
$ascii = unpack('c*',$s);
print_r($ascii);
Array
(
    [1] => 112
    [2] => 108
    [3] => 97
    [4] => 116
    [5] => 121
    [6] => 112
    [7] => 117
    [8] => 115
)


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