PHP Manual: Pack data into binary string

Description

string pack(string $format, mixed ...$values)

Pack given arguments into a binary string according to format.

The idea for this function was taken from Perl and all formatting codes work the same as in Perl. However, there are some formatting codes that are missing such as Perl's "u" format code.

Note that the distinction between signed and unsigned values only affects the function unpack, where as function pack gives the same result for signed and unsigned format codes.

Parameters

format

The format string consists of format codes followed by an optional repeater argument. The repeater argument can be either an integer value or * for repeating to the end of the input data. For a, A, h, H the repeat count specifies how many characters of one data argument are taken, for @ it is the absolute position where to put the next data, for everything else the repeat count specifies how many data arguments are consumed and packed into the resulting binary string.

Currently implemented formats are:

**pack format characters**
Code	Description
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 (always 16 bit, machine byte order)
S	unsigned short (always 16 bit, machine byte order)
n	unsigned short (always 16 bit, big endian byte order)
v	unsigned short (always 16 bit, little endian byte order)
i	signed integer (machine dependent size and byte order)
I	unsigned integer (machine dependent size and byte order)
l	signed long (always 32 bit, machine byte order)
L	unsigned long (always 32 bit, machine byte order)
N	unsigned long (always 32 bit, big endian byte order)
V	unsigned long (always 32 bit, little endian byte order)
q	signed long long (always 64 bit, machine byte order)
Q	unsigned long long (always 64 bit, machine byte order)
J	unsigned long long (always 64 bit, big endian byte order)
P	unsigned long long (always 64 bit, little endian byte order)
f	float (machine dependent size and representation)
g	float (machine dependent size, little endian byte order)
G	float (machine dependent size, big endian byte order)
d	double (machine dependent size and representation)
e	double (machine dependent size, little endian byte order)
E	double (machine dependent size, big endian byte order)
x	NUL byte
X	Back up one byte
Z	NUL-padded string
@	NUL-fill to absolute position

values

Return Values

Returns a binary string containing data.

Changelog

Version	Description
8.0.0	This function no longer returns `false` on failure.
7.2.0	float and double types supports both Big Endian and Little Endian.
7.0.15, 7.1.1	The "e", "E", "g" and "G" codes were added to enable byte order support for float and double.

Examples

Example #1 pack example

<?php
$binarydata = pack("nvc*", 0x1234, 0x5678, 65, 66);
?>

The resulting binary string will be 6 bytes long and contain the byte sequence 0x12, 0x34, 0x78, 0x56, 0x41, 0x42.

Notes

Caution

Note that PHP internally stores int values as signed values of a machine-dependent size (C type long). Integer literals and operations that yield numbers outside the bounds of the int type will be stored as float. When packing these floats as integers, they are first cast into the integer type. This may or may not result in the desired byte pattern.

The most relevant case is when packing unsigned numbers that would be representable with the int type if it were unsigned. In systems where the int type has a 32-bit size, the cast usually results in the same byte pattern as if the int were unsigned (although this relies on implementation-defined unsigned to signed conversions, as per the C standard). In systems where the int type has 64-bit size, the float most likely does not have a mantissa large enough to hold the value without loss of precision. If those systems also have a native 64-bit C int type (most UNIX-like systems don't), the only way to use the I pack format in the upper range is to create int negative values with the same byte representation as the desired unsigned value.

pack