PHP's pack() and unpack() functions are used to convert between PHP variables and binary data. 1. pack() packages variables such as integers and strings into binary data, and unpack() unpacks the binary data into PHP variables. Both rely on format strings to specify conversion rules. 2. Common format codes include C/c (8-bit with/unsigned characters), S/s (16-bit short integer), L/l/V/N (32-bit long integer, corresponding to different endianness), f/d (floating point/double precision), a/A (fill string), x (null byte), etc. 3. Endite order is crucial: V represents little-endian (Intel), N represents big-endian (network standard). V or N should be used first when communicating across platforms to ensure consistency. 4. In practical applications, unpack() can parse BMP file headers, such as unpack('C2signature/Vsize/Vreserved/Voffset', $binary) to read 14-byte BMP headers and name fields; pack() can be used to build custom binary protocols, such as pack('Cv', $type, $length) to generate message headers. 5. Notes include: confirming the byte order of the data, distinguishing whether there are signs, correctly handling string filling methods (a is filled with NULL, A is filled with spaces), and naming the fields through /name syntax to improve readability. 6. These functions are suitable for parsing binary file formats such as ZIP and PNG, communicating with hardware, implementing network protocols or processing C structure data, and providing fine control of the underlying data without expansion. As long as the format string accurately matches the data structure, binary data operations can be reliably performed.

Working with binary data in PHP might sound like something only low-level systems programs deal with, but it's actually useful in many practical scenarios—like parsing file formats (PNG, ZIP), handling network protocols, or communicating with hardware. PHP's pack() and unpack() functions are powerful tools for converting between data types and their binary representations.

Let's break down how they work and when to use them—without the jargon overload.

What Do pack() and unpack() Actually Do?

At their core:

• pack() converts PHP variables (like integers, strings) into binary data based on a format string.
• unpack() reads binary data and converts it back into PHP variables, again using a format string.

They're like translators between PHP's high-level world and the raw bytes that computers and networks speak.

 $data = pack('S', 258); // Convert integer 258 to 2-byte unsigned short
$unpacked = unpack('S', $data); // Convert back
var_dump($unpacked); // [1] => 258

Note: unpack() returns an array, even for a single value.

Understanding the Format Codes

The magic (and confusion) lies in the format string —a sequence of letters and numbers telling PHP how to interpret the data.

Here are the most commonly used codes:

Note on endianness :

• V = Little-endian (Intel, most common)
• N = Big-endian (network byte order)
• L = Machine-dependent (usually little-endian)

If you're dealing with cross-platform or network data, prefer N and V for predictable behavior.

Practical Example: Reading a BMP File Header

Let's say you want to read basic info from a BMP file. The first 14 bytes are the file header.

 $file = fopen('image.bmp', 'rb');
$binary = fread($file, 14);
fclose($file);

// Unpack BMP header
$header = unpack('C2signature/Vsize/Vreserved/Voffset', $binary);

// Results:
// $header['signature1'] = 66 ('B')
// $header['signature2'] = 77 ('M')
// $header['size'] = file size in bytes
// $header['offset'] = where pixel data starts

Here:

• C2signature reads two bytes as unsigned chars and names them signature1 , signature2
• Vsize reads a 32-bit unsigned long in little-endian (BMP uses little-endian)
• You can name fields by adding /name after the format code

This makes it easy to parse structured binary formats.

Packing Data: Building a Binary Message

Imagine you're creating a simple binary protocol where each message contains:

• Message type (1 byte)
• Length (2 bytes, little-endian)
• Payload (string)

 $messageType = 1;
$payload = 'Hello';
$length = strlen($payload);

$binary = pack('Cv', $messageType, $length) . $payload;

Here:

• C = message type (unsigned char)
• v = length as unsigned short, little-endian (lowercase = little-endian)

Now $binary can be sent over a socket or saved to a file.

To read it back:

 $data = unpack('Ctype/vlength', $binary);
$payload = substr($binary, 3, $data['length']);

Tips and Gotchas

• Endianness matters : When working with hardware or network protocols, always confirm byte order. Use N (big-endian) or V (little-endian) for consistency.
• Signed vs. unsigned : c vs C , s vs S —mixing them up leads to negative numbers or overflow.
• String padding : a pads with NULLs, A with spaces. Use a for binary-safe strings.
• Field naming : Use /name syntax to make results readable:

   unpack('Cversion/Ctype/a10data', $bin);
  // returns ['version'=>1, 'type'=>2, 'data'=>"payload..."]

• unpack() returns indexed arrays by default unless you name the fields.

---

When Should You Use These Functions?

• Parsing binary file formats: ZIP, PNG, WAV, etc.
• Communicating with embedded devices or sensors
• Creating or reading custom binary protocols
• Handling packed data from C structures or databases

They're not everyday tools, but when you need them, there's no real alternative in pure PHP.

---

Basically, pack() and unpack() give you fine-grained control over binary data without needing extensions or external libraries. It's not flashy, but once you understand the format codes, you can interface with low-level data structures directly in PHP.

Just remember: match your format string to the data's structure and byte order—get that right, and the rest follows.

The above is the detailed content of Unpacking Binary Data: A Practical Guide to PHP's `pack()` and `unpack()`. For more information, please follow other related articles on the PHP Chinese website!