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 )

not sure why, but you could run array_values to re-index the keys if it is really a problem — mic
– mic, Commented Dec 23, 2013 at 15:41
Your code is trying to convert a string to binary data, hence you have to use pack, not unpack — Hanky Panky
– Hanky Panky, Commented Dec 23, 2013 at 15:44
I tried with this($str = "0x123";) binary data, this also returns with array index 1. — user2644574
– user2644574, Commented Dec 23, 2013 at 15:45
@Hanky웃Panky Unpacking the string "PHP" is a legitimate test of unpack(). There's no reason to believe the OP meant to pack rather than unpack. — John Kugelman
– John Kugelman, Commented Dec 23, 2013 at 15:53

Community · Accepted Answer · 2020-06-20 09:12:55Z

5

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".

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Dec 23, 2013 at 15:43

John Kugelman

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1 Comment

nanaya Over a year ago

This doesn't really answer why it starts from 1 instead of 0 (chars1 instead of chars0 etc in the provided example). Also all arrays in php are technically associative.

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