pack() function in PHP

The pack() function in PHP converts data into a binary string according to a specified format. This function is useful for creating binary data structures, network protocols, or file formats that require specific byte arrangements.

Syntax

pack(format, args)

Parameters

• format − The format string specifying how to pack the data. Common format codes include:

  • a − NUL-padded string

  • A − SPACE-padded string

  • c − signed char (8-bit)

  • C − unsigned char (8-bit)

  • s − signed short (16-bit, machine byte order)

  • S − unsigned short (16-bit, machine byte order)

  • i − signed integer (machine dependent)

  • I − unsigned integer (machine dependent)

  • l − signed long (32-bit, machine byte order)

  • L − unsigned long (32-bit, machine byte order)

  • f − float (machine dependent)

  • d − double (machine dependent)

• args − One or more values to be packed according to the format

Return Value

Returns a binary string containing the packed data.

Basic Example

Here's how to pack three unsigned characters into a binary string −

<?php
   $binary = pack("C3", 80, 72, 80);
   echo $binary;
?>

PHP

String Packing Example

Packing strings with different padding methods −

<?php
   // NUL-padded string
   $packed1 = pack("a10", "Hello");
   echo "NUL-padded: " . bin2hex($packed1) . "<br>";
   
   // SPACE-padded string
   $packed2 = pack("A10", "Hello");
   echo "SPACE-padded: " . bin2hex($packed2) . "<br>";
?>

NUL-padded: 48656c6c6f0000000000
SPACE-padded: 48656c6c6f2020202020

Integer Packing Example

Packing different integer types −

<?php
   // Pack as unsigned short (16-bit)
   $short = pack("S", 1234);
   echo "Short (16-bit): " . bin2hex($short) . "<br>";
   
   // Pack as unsigned long (32-bit)
   $long = pack("L", 123456);
   echo "Long (32-bit): " . bin2hex($long) . "<br>";
?>

Short (16-bit): d204
Long (32-bit): 40e20100

Conclusion

The pack() function is essential for binary data manipulation in PHP. Use format codes like "C" for unsigned chars, "S" for shorts, and "L" for longs to create structured binary data for protocols or file formats.