Why Unpack function returns array with starting index 1 in PHP

Question

Why unpack() function in PHP returns an array of binary data starting with the array index 1.

$str = "PHP";
$binary_data = unpack("C*",$str);
print_r($binary_data);

The above PHP scripts prints as below:

Array ( [1] => 80 [2] => 72 [3] => 80 )

Answer 1

The array is an associative array with named keys, not a regular array with numeric keys. The idea is that you will name each format code and the result array will use those names as the array keys.

For example:

<?php
$str = "PHP";
$binary_data = unpack("C*letter",$str);
print_r($binary_data);

Result:

Array
(
    [letter1] => 80
    [letter2] => 72
    [letter3] => 80
)

From the PHP manual:

The unpacked data is stored in an associative array. To accomplish this you have to name the different format codes and separate them by a slash /. If a repeater argument is present, then each of the array keys will have a sequence number behind the given name.

Example #1 unpack() example

<?php
$binarydata = "\x04\x00\xa0\x00";
$array = unpack("cchars/nint", $binarydata);
?>

The resulting array will contain the entries "chars" with value 4 and "int" with 160.

Example #2 unpack() example with a repeater

<?php
$binarydata = "\x04\x00\xa0\x00";
$array = unpack("c2chars/nint", $binarydata);
?>

The resulting array will contain the entries "chars1", "chars2" and "int".