Use pack() to implement serialization of binary data

In PHP, the pack() function is used to package data into binary strings in the specified format. This is useful when dealing with network transfers, file writing, and interactions with C or other low-level languages. This article will explain in detail the usage of the pack() function and combine examples to illustrate how to package data into binary format.

Pack() function introduction

The basic syntax of pack() is as follows:

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

• $format : Specifies the format of the data, consisting of one or more format codes.

• $values : The value to be packaged can be multiple parameters.

The return value is a binary string packaged in the specified format.

Common format codes

Example analysis

1. Packaging strings and integers

 <?php
// Pack one length4string and one32bit unsigned big endian integer
$packed = pack('a4N', 'test', 123456789);
echo bin2hex($packed);
?>

Here 'a4' means packing a string of length 4, the insufficient part is filled with NUL characters; 'N' means packing a 32-bit unsigned big-endian integer. bin2hex() converts binary content into hexadecimal for easy viewing.

2. Use pack() to build network protocol packages

Suppose you need to construct a simple network packet, including the command type (1 byte) and the data length (4 bytes big endian), and then the data body:

 <?php
$command = 1;               // Command Type，1byte
$data = "Hello, m66.net!";  // Data body
$length = strlen($data);    // Data length

// 打包Command Type和Data length
$header = pack('CN', $command, $length);

// Splicing complete data packets
$packet = $header . $data;

// Hexadecimal representation of output package
echo bin2hex($packet);
?>

In the above code, 'C' package command type (1 byte unsigned integer), and 'N' package data length (4 byte big-endian integer). Data bodies are spliced ​​as is.

3. Example of using pack() and unpack() to convert each other

 <?php
$number = 65535;
$packed = pack('n', $number);  // 按大端byte序打包为16Unsigned integer
echo 'Packed data: ' . bin2hex($packed) . PHP_EOL;

$unpacked = unpack('nvalue', $packed);  // Unpacking
echo 'Unpacked value: ' . $unpacked['value'] . PHP_EOL;
?>

summary

pack() is a powerful tool for manipulating binary data, especially suitable for network programming and underlying data processing. By skilled in using format code, you can accurately control the byte arrangement and type of data.