pack

Pack data into binary string

&reftitle.description; `string` `pack` `string` `format` `mixed` `args` `mixed``...`

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.

&reftitle.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)
  f | float (machine dependent size and representation)
  d | double (machine dependent size and representation)
  x | NUL byte
  X | Back up one byte
  Z | NUL-padded string (new in PHP 5.5)
  @ | NUL-fill to absolute position

`args`

&reftitle.returnvalues;

Returns a binary string containing data.

&reftitle.changelog;

&Version; | &Description;
5.5.0 | The "Z" code was added with equivalent functionality to "a" for Perl compatibility.

&reftitle.examples;

### `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.

&reftitle.notes;

Note that PHP internally stores `integer` values as signed values of a machine-dependent size (C type `long`). Integer literals and operations that yield numbers outside the bounds of the `integer` 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 `integer` type if it were unsigned. In systems where the `integer` type has a 32-bit size, the cast usually results in the same byte pattern as if the `integer` were unsigned (although this relies on implementation-defined unsigned to signed conversions, as per the C standard). In systems where the `integer` 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 `integer` negative values with the same byte representation as the desired unsigned value.

&reftitle.seealso;

- `unpack`