How to use PHP's pack() function and unpack() format code? A list of complete comparison tables

M66 2025-05-26

In PHP, pack() and unpack() functions are often used to process binary data, and are especially suitable for use in scenarios such as network communication, file reading and writing, data encoding, etc. where precise control of bytes is required. This article will explain in detail the usage of these two functions and attach a complete format code comparison table to help you process binary data more efficiently.

1. What are pack() and unpack() ?

pack() : Package data into binary string.
unpack() : Unpack the binary string into a PHP array.

Both use format codes to define the structure of the data, which determine how the data is encoded/decoded.

2. Usage of pack() function

 $data = pack("C*", 72, 101, 108, 108, 111); // Will ASCII Convert code to string "Hello"
echo $data; // Output: Hello

grammar:

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

3. Usage of unpack() function

 $data = "Hello";
$result = unpack("C*", $data);
print_r($result);

Output:

 Array
(
    [1] => 72
    [2] => 101
    [3] => 108
    [4] => 108
    [5] => 111
)

grammar:

 array unpack(string $format, string $data, int $offset = 0)

4. Common format code comparison table

Format code	illustrate	Byte length	Large and small end
a	NUL Fill the string (not removing NUL)	Specify length	none
A	Spaces fill strings (the tail spaces will be removed)	Specify length	none
h	Hexadecimal string, each character represents 4 bits	Every two digits = 1 byte	none
H	Caps hexadecimal, each character indicates 4 bits	Every two digits = 1 byte	none
c	Signed characters (8 bits)	1 byte	none
C	Unsigned characters (8 bits)	1 byte	none
s	Signed short integer (16 bits)	2 bytes	Host byte order
S	Unsigned short integer (16-bit)	2 bytes	Host byte order
n	Unsigned short integer (16-bit), network byte order	2 bytes	Big endian
v	Unsigned short integer (16-bit), little-endian byte order	2 bytes	Little endian
i	Signed integers (machine-related)	4 bytes or 8 bytes	Host byte order
I	Unsigned integers (machine-related)	4 bytes or 8 bytes	Host byte order
l	Signed long integer (32 bits)	4 bytes	Host byte order
L	Unsigned long (32-bit)	4 bytes	Host byte order
N	Unsigned long integer (32 bits), network byte order	4 bytes	Big endian
V	Unsigned long integer (32 bits), little endian byte order	4 bytes	Little endian
f	Single-precision floating point number (32 bits)	4 bytes	Host byte order
d	Double precision floating point number (64 bits)	8 bytes	Host byte order
x	Fill in bytes (skip 1 byte)	1 byte	none
X	Rewind 1 byte	1 byte	none
@	Set the absolute position (skip/fall back to a position)	variable	none

5. Practical application examples

Network communication: IP address conversion

 $ip = "192.168.1.1";
$packed = pack("C4", ...explode('.', $ip));
echo bin2hex($packed); // Output c0a80101

$unpacked = unpack("C4", $packed);
echo implode('.', $unpacked); // Output 192.168.1.1

Binary protocol parsing (such as reading file headers)

 $binary = file_get_contents("https://m66.net/example/file.bin");
$header = unpack("a4signature/Nsize", $binary);
print_r($header);

6. Quantity modifier for format code

The format code can be modified with quantity, for example:

C4 : represents 4 unsigned characters
A10 : padded string with spaces representing 10 characters
@8 : Jump to the 8th byte position

You can also use * to read as much as possible:

 $data = unpack("C*", "Hello"); // Read all bytes

7. Summary

PHP's pack() and unpack() are powerful binary processing tools. Being familiar with its format code and usage can help you process underlying data more flexibly. Whether it is parsing protocols, storing structured data, or building efficient network communication protocols, these two functions are very practical tools.

Although there are many format codes, mastering the commonly used types (such as C , n , N , f , d ) can meet most needs. It is recommended to use bin2hex() and hex2bin() together in development, for debugging is more convenient.

pack
unpack