Can someone explain me the pack() function in PHP?

Question

Hi,

I would like to know more about the pack() function in PHP: http://fi.php.net/manual/en/function.pack.php

I know it packs data into binary, but I'm not sure what all those v V n N c C mean and I was wondering if someone could be kind and give me a practical demonstration when to use which formats?

The online documentation, for change, lacks of information, in my opinion.

Answer 1

Those represent how you want the data you are packing to be represented in binary format:

so $bin = pack("v", 1); => 0000000000000001 (16bit)

where

$bin = pack("V", 1) => 00000000000000000000000000000001 (32 bit)

It tells pack how you want the data represented in the binary data. The code below will demonstrate this. Note that you can unpack with a different format from what you packed the data as.

<?php

$bin = pack("S", 65535);
$ray = unpack("S", $bin);
echo "UNSIGNED SHORT VAL = ", $ray[1], "\n";

$bin = pack("S", 65536);
$ray = unpack("S", $bin);
echo "OVERFLOW USHORT VAL = ", $ray[1], "\n";

$bin = pack("V", 65536);
$ray = unpack("V", $bin);
echo "SAME AS ABOVE BUT WITH ULONG VAL = ", $ray[1], "\n";

?>

Answer 2

As noted in the php documentation for pack, the function is borrowed from Perl's pack function.

Take a look at Perl's documentation for pack, specifically the examples section at the very bottom of the page. PHP's pack does not implement all the formats, but Perl's documentation for the function does a better job of providing examples and explaining each format.