unpack

(PHP 4, PHP 5)

unpackUnpack data from binary string

Descrizione

array unpack ( string $format , string $data )

Unpacks from a binary string into an array according to the given format.

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.

Elenco dei parametri

format

See pack() for an explanation of the format codes.

data

The packed data.

Valori restituiti

Returns an associative array containing unpacked elements of binary string.

Log delle modifiche

Versione Descrizione
5.5.0

Changes were made to bring this function into line with Perl:

The "a" code now retains trailing NULL bytes.

The "A" code now strips all trailing ASCII whitespace (spaces, tabs, newlines, carriage returns, and NULL bytes).

The "Z" code was added for NULL-padded strings, and removes trailing NULL bytes.

Esempi

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".

Note

Attenzione

Note that PHP internally stores integral values as signed. If you unpack a large unsigned long and it is of the same size as PHP internally stored values the result will be a negative number even though unsigned unpacking was specified.

Attenzione

Be aware that if you do not name an element, an empty string is used. If you do not name more than one element, this means that some data is overwritten as the keys are the same such as in:

Example #3 unpack() example with unnamed keys

<?php
$binarydata 
"\x32\x42\x00\xa0";
$array unpack("c2/n"$binarydata);
var_dump($array);
?>

The resulting array will contain the entries "1" with value 160 and "2" with 66. The first value from the c specifier is overwritten by the first value from the n specifier.

Vedere anche:

  • pack() - Pack data into binary string

add a note add a note

User Contributed Notes 24 notes

up
6
Sergio Santana: ssantana at tlaloc dot imta dot mx
10 years ago
This is about the last example of my previous post. For the sake of clarity, I'm including again here the example, which expands the one given in the formal documentation:

<?
  $binarydata = "AA\0A";
  $array = unpack("c2chars/nint", $binarydata);
  foreach ($array as $key => $value)
     echo "\$array[$key] = $value <br>\n";
?>

This outputs:

$array[chars1] = 65
$array[chars2] = 65
$array[int] = 65

Here, we assume that the ascii code for character 'A' is decimal 65.

Remebering that the format string structure is:
<format-code> [<count>] [<array-key>] [/ ...],
in this example, the format string instructs the function to
  1. ("c2...") Read two chars from the second argument ("AA ...),
  2. (...chars...) Use the array-keys "chars1", and "chars2" for
      these two chars read,
  3. (.../n...) Read a short int from the second argument (...\0A"),
  4. (...int") Use the word "int" as the array key for the just read
      short.

I hope this is clearer now,

Sergio.
up
4
jjfoerch at earthlink dot net
9 years ago
I had a situation where I had to unpack a file filled with little-endian order double-floats in a way that would work on either little-endian or big-endian machines.  PHP doesn't have a formatting code that will change the byte order of doubles, so I wrote this workaround.

<?php
/*The following code is a workaround for php's unpack function
which does not have the capability of unpacking double precision
floats that were packed in the opposite byte order of the current
machine.
*/
function big_endian_unpack ($format, $data) {
   
$ar = unpack ($format, $data);
   
$vals = array_values ($ar);
   
$f = explode ('/', $format);
   
$i = 0;
    foreach (
$f as $f_k => $f_v) {
   
$repeater = intval (substr ($f_v, 1));
    if (
$repeater == 0) $repeater = 1;
    if (
$f_v{1} == '*')
    {
       
$repeater = count ($ar) - $i;
    }
    if (
$f_v{0} != 'd') { $i += $repeater; continue; }
   
$j = $i + $repeater;
    for (
$a = $i; $a < $j; ++$a)
    {
       
$p = pack ('d',$vals[$i]);
       
$p = strrev ($p);
        list (
$vals[$i]) = array_values (unpack ('d1d', $p));
        ++
$i;
    }
    }
   
$a = 0;
    foreach (
$ar as $ar_k => $ar_v) {
   
$ar[$ar_k] = $vals[$a];
    ++
$a;
    }
    return
$ar;
}

list (
$endiantest) = array_values (unpack ('L1L', pack ('V',1)));
if (
$endiantest != 1) define ('BIG_ENDIAN_MACHINE',1);
if (
defined ('BIG_ENDIAN_MACHINE')) $unpack_workaround = 'big_endian_unpack';
else
$unpack_workaround = 'unpack';
?>

This workaround is used like this:

<?php

function foo() {
        global
$unpack_workaround;
   
$bar = $unpack_workaround('N7N/V2V/d8d',$my_data);
//...
}

?>

On a little endian machine, $unpack_workaround will simply point to the function unpack.  On a big endian machine, it will call the workaround function.

Note, this solution only works for doubles.  In my project I had no need to check for single precision floats.
up
3
Sergio Santana: ssantana at tlaloc dot imta dot mx
10 years ago
Suppose we need to get some kind of internal representation of an integer, say 65, as a four-byte long. Then we use, something like:

<?
  $i = 65;
  $s = pack("l", $i); // long 32 bit, machine byte order
  echo strlen($s) . "<br>\n";
  echo "***$s***<br>\n";
?>

The output is:

X-Powered-By: PHP/4.1.2
Content-type: text/html

4
***A***

(That is the string "A\0\0\0")

Now we want to go back from string "A\0\0\0" to number 65. In this case we can use:

<?
  $s = "A\0\0\0"; // This string is the bytes representation of number 65
  $arr = unpack("l", $s);
  foreach ($arr as $key => $value)
     echo "\$arr[$key] = $value<br>\n";
?>

And this outpus:
X-Powered-By: PHP/4.1.2
Content-type: text/html

$arr[] = 65

Let's give the array key a name, say "mykey". In this case, we can use:

<?
  $s = "A\0\0\0"; // This string is the bytes representation of number  65
  $arr = unpack("lmykey", $s);
  foreach ($arr as $key => $value)
     echo "\$arr[$key] = $value\n";
?>

An this outpus:
X-Powered-By: PHP/4.1.2
Content-type: text/html

$arr[mykey] = 65

The "unpack" documentation is a little bit confusing. I think a more complete example could be:

<?
  $binarydata = "AA\0A";
  $array = unpack("c2chars/nint", $binarydata);
  foreach ($array as $key => $value)
    echo "\$array[$key] = $value <br>\n";
?>

whose output is:

X-Powered-By: PHP/4.1.2
Content-type: text/html

$array[chars1] = 65 <br>
$array[chars2] = 65 <br>
$array[int] = 65 <br>

Note that the format string is something like
<format-code> [<count>] [<array-key>] [/ ...]

I hope this clarifies something

Sergio
up
1
David Gero dave at havidave dot com
3 years ago
You might find these functions useful:

<?php
function byteStr2byteArray($s) {
        return
array_slice(unpack("C*", "\0".$s), 1);
}
function
byteArray2byteStr(array $t) {
        return
call_user_func_array(pack, array_merge(array("C*"), $t));
}
function
lsbStr2ushortArray($s) {
        return
array_slice(unpack("v*", "\0\0".$s), 1);
}
function
ushortArray2lsbStr(array $t) {
        return
call_user_func_array(pack, array_merge(array("v*"), $t));
}
function
lsbStr2ulongArray($s) {
        return
array_slice(unpack("V*", "\0\0\0\0".$s), 1);
}
function
ulongArray2lsbStr(array $t) {
        return
call_user_func_array(pack, array_merge(array("V*"), $t));
}
?>

Of course, you can address byte strings as if they're arrays with numerical indexes, but the other functions are helpful.
up
1
Nhon
6 years ago
As stated above, "if you unpack a large unsigned long and it is of the same size as PHP internally stored values the result will be a negative number even though unsigned unpacking was specified."

To restore the original unsigned value, you could do this :

if ($unpackedVal <0)
{
      $unpackedVal += 4294967296;
}

Hope this helps !

Cheers
up
2
kennwhite dot nospam at hotmail dot com
9 years ago
If having a zero-based index is useful/necessary, then instead of:

$int_list = unpack("s*", $some_binary_data);

try:

$int_list = array_merge(unpack("s*", $some_binary_data));

This will return a 0-based array:

$int_list[0] = x
$int_list[1] = y
$int_list[2] = z
...

rather than the default 1-based array returned from unpack when no key is supplied:

$int_list[1] = x
$int_list[2] = y
$int_list[3] = z
...

It's not used often, but array_merge() with only one parameter will compress a sequentially-ordered numeric-index, starting with an index of [0].
up
1
rogier
2 years ago
be aware of the behavior of your system that PHP resides on.

On x86, unpack MAY not yield the result you expect for UInt32

This is due to the internal nature of PHP, being that integers are internally stored as SIGNED!

For x86 systems, unpack('N', "\xff\xff\xff\xff") results in -1
For (most?) x64 systems, unpack('N', "\xff\xff\xff\xff") results in 4294967295.

This can be verified by checking the value of PHP_INT_SIZE.
If this value is 4, you have a PHP that internally stores 32-bit.
A value of 8 internally stores 64-bit.

To work around this 'problem', you can use the following code to avoid problems with unpack.
The code is for big endian order but can easily be adjusted for little endian order (also, similar code works for 64-bit integers):

<?php
function _uint32be($bin)
{
   
// $bin is the binary 32-bit BE string that represents the integer
   
if (PHP_INT_SIZE <= 4){
        list(,
$h,$l) = unpack('n*', $bin);
        return (
$l + ($h*0x010000));
    }
    else{
        list(,
$int) = unpack('N', $bin);
        return
$int;
    }
}
?>

Do note that you *could* also use sprintf('%u', $x) to show the unsigned real value.
Also note that (at least when PHP_INT_SIZE = 4) the result WILL be a float value when the input is larger then 0x7fffffff (just check with gettype);

Hope this helps people.
up
1
zac at picolink dot net
3 years ago
The documentation is clear that an integer read using an unsigned format character will still be stored as a signed integer.  The often-cited work-around is to use sprintf('%u', $bigint) to properly display integers with the MSB set.

In the case where the numeric value is more important than how it's displayed, you can still work with other large integers using intval() to "upgrade" your existing unsigned integers.

I had a problem comparing 32-bit integers read from files with hard-coded constants (file signatures tend to need this).  Here's what I did to avoid converting everything into strings:

<?php

$bigint
= 0x89504E47;

$packed = pack('N', $bigint);

list(
$unpacked) = array_values(unpack('N', $packed));

//The $bigint remains an unsigned integer.
//Even though their bit-wise values are identical, comparison fails.

echo 'bigint ',
  (
$bigint == $unpacked ? '==' : '!='),
 
" unpacked\n";

//intval() triggers a re-interpretation of $bigint.
//$bigint is internally compared as a signed integer.
//Since the bit-wise value of $bigint never changes, comparison succeeds.

echo 'intval(bigint) ',
  (
intval($bigint) == $unpacked ? '==' : '!='),
 
" unpacked\n";

?>

It works, but it's a little backwards.  If anyone has any ideas on how to "downgrade" a signed integer into an unsigned integer without using strings, that would be a valuable note to add to the documentation.
up
1
jlarsen at fsu dot edu
5 years ago
As with perl, the count for hex is number of nybbles or half-bytes, this differs from the other options which count in full bytes.
up
0
yvan dot burrie at hotmail dot com
9 months ago
Here's a demonstration concerning the speed of unpacking files:
So let's see which method is fastest between FREAD or SUBSTR?

I was creating a script that could read scenario files from a game, and render a preview of its terrain. The terrain structure within each file was huge (between 100,000 - 1,000,000 blocks containing 3 bits of data each). Therefore, I spent much effort to ensure it was fast and robust.

Method 1: This method retrieves the 3 bits of data found in each block. It uses the loop of widthxheight and implode+unpack+substr each block:
<?php
for ( $Y = 0 ; $Y < ( $width * $height ) ; $Y ++ ) {
   
$Output [ Map ] [ $Y ] [ TerrainID ] = implode ( null , unpack ( 'c1' , substr ( $Input , $Line ) ) ) ; $Line += 1 ;
   
$Output [ Map ] [ $Y ] [ Elevation ] = implode ( null , unpack ( 'c1' , substr ( $Input , $Line ) ) ) ; $Line += 1 ;
   
$Output [ Map ] [ $Y ] [ Unknown ] = implode ( null , unpack ( 'c1' , substr ( $Input , $Line ) ) ) ; $Line += 1 ;
}
//The average microtime was: 2.9 sec
?>
Note that it takes even more time if you use a custom function to implement the implode+unpack+substr functions.

Now... This method uses the FREAD function:
<?php
for ( $Y = 0 ; $Y < ( $width * $height ) ; $Y ++ ) {
   
$Output [ Map ] [ $Y ] = unpack ( 'c3' , fread ( $sc , 3 ) ) ;
}
//Average microtime was: 0.7 sec
?>
I recommend using the FREAD method instead of SUBSTR.

Another test!!! This method is 10x faster than the above. This does not use the FOR loop:
<?php
$Output
[ Map ] [ Data ] = unpack ( 'c' . ( $width * $height ) , stream_get_contents ( $sc ) ) ;
//Average microtime: 0.08 - 0.05 sec
?>

If you want to read files much faster, you should try to reduce the number of loops and use the unpack function to its simplest and robust method.
up
0
googlybash24 at aol dot com
1 year ago
To convert big endian to little endian or to convert little endian to big endian, use the following approach as an example:

<?php
// file_get_contents() returns a binary value, unpack("V*", _ ) returns an unsigned long 32-bit little endian decimal value, but bin2hex() after that would just give the hex data in the file if alone, so instead we use:
// file_get_contents(), unpack("V*", _ ), then dechex(), in that order, to get the byte-swapping effect.
?>

With the logic of the approach in this example, you can discover how to swap the endian byte order as you need.
up
0
Aaron Wells
3 years ago
Another option for converting binary data into PHP data types, is to use the Zend Framework's Zend_Io_Reader class:
http://bit.ly/9zAhgz

There's also a Zend_Io_Writer class that does the reverse.
up
0
norwood at computer dot org
4 years ago
Reading a text cell from an Excel spreadsheet returned a string with low-order embedded nulls: 0x4100 0x4200 etc. To remove the nulls, used

<?php
$strWithoutNulls
= implode( '', explode( "\0", $strWithNulls ) );
?>

(unpack() didn't seem to help much here; needed chars back to re-constitute the string, not integers.)
up
0
Anonymous
4 years ago
Functions I found useful when dealing with fixed width file processing, related to unpack/pack functions.
<?php
/**
* funpack
* format: array of key, length pairs
* data: string to unpack
*/
function funpack($format, $data){
    foreach (
$format as $key => $len) {
       
$result[$key] = trim(substr($data, $pos, $len));
       
$pos+= $len;
    }
    return
$result;
}

/**
* fpack
* format: array of key, length pairs
* data: array of key, value pairs to pack
* pad: padding direction
*/
function fpack($format, $data, $pad = STR_PAD_RIGHT){
    foreach (
$format as $key => $len){
       
$result .= substr(str_pad($data[$key], $len, $pad), 0, $len);
    }
    return
$result;
}
?>
up
0
sica at wnet com br
5 years ago
The script following is a example how to save more than one values on file separating its with "\r\n" and how to recovering its values.

<?php
// Save two integer values in a binary file
$nomearq = "./teste.bin";
$valor = 123;
$ptrarq = fopen($nomearq, "wb");
$valorBin = pack("L",$valor);
echo
"First value ($valor) packed with ";
echo
fwrite($ptrarq, $valorBin)." bytes<br>";
echo
"Separator \\r\\n with ";
echo
fwrite($ptrarq, "\r\n")." bytes<br>";
$valor = 456;
$valorBin = pack("L",$valor);
echo
"Second value ($valor) packed with ";
echo
fwrite($ptrarq, $valorBin)." bytes<br>";
fclose($ptrarq);

// Recover the saved values
$ptrarq = fopen($nomearq, "rb");
$valorBin = file($nomearq,filesize($nomearq));
echo
"<br>The reading values is:<br>";
foreach(
$valorBin as $valor){
 
$valor = unpack("L",$valor);
 
print_r($valor);
  echo
"<br>";
}
fclose($ptrarq);
?>

Results:
First value (123) packed with 4 bytes
Separator \r\n with 2 bytes
Second value (456) packed with 4 bytes

The reading values is:
Array ( [1] => 123 )
Array ( [1] => 456 )
up
0
Anonymous Coward
6 years ago
Warning: This unpack function makes the array with keys starting at 1 instead of starting at 0.

For example:
<?php
function read_field($h) {
 
$a=unpack("V",fread($h,4));
  return
fread($h,$a[1]);
}
?>
up
0
joe dot nemeth @ palg dot com
6 years ago
A simpler solution is to mask the value with 0xffffffff. For instance:

<?php
$rec
= unpack(
 
"Vvalue/".
 
"Vhash32/",
 
$recbin);
$rec['hash32'] &= 0xffffffff;
$rec['value'] &= 0xffffffff;
?>

Unlike sprintf(), which converts the value to a string, this preserves the numeric type of the value.
up
0
Shawn Kelly
6 years ago
Above it says this:

  "Note that PHP internally stores integral values as signed. If  you unpack a large unsigned long and it is of the same size as PHP internally stored values the result will be a negative number even though unsigned unpacking was specified."

This happened to me.  I wanted to get a big number from a unsigned long, but it kept coming returning a negative.  Happened to notice that sprintf('%u',$dta) will take the useless negative and restore it into its large unsigned proper magnitude.

Hope this saves someone a little time...
up
0
adam at adeptsoftware dot com
12 years ago
If you just want to extract a dword/long int from a binary string, the following code works beautifully (intel endian):

$Number = ord($Buffer{0}) | (ord($Buffer{1})<<8) | (ord($Buffer{2})<<16) | (ord($Buffer{3})<<24);
up
0
DanRichter.at.programmer.dot.net
13 years ago
If no key name is given [e.g., unpack('C*',$data)], the keys are simply integers starting at 1, and you have a standard array. (I know of no way to get the array to start at zero.)

If you use multiple types, you must give a key name for all of them (except optionally one), because the key counter is reset with each slash. For example, in unpack('n2/C*',$data), indices 1 and 2 of the returned array are filled by integers ('n'), then overwritten with characters ('C').
up
0
iredden at redden dot on dot ca
14 years ago
<?php

function parse_pascalstr($bytes_parsed, $parse_str) {
   
$parse_info = unpack("x$bytes_parsed/cstr_len", $parse_str);
   
$str_len = $parse_info["str_len"];
   
$bytes_parsed = $bytes_parsed + 1;
   
$parse_info = unpack("x$bytes_parsed/A".$str_len."str", $parse_str);
   
$str = $parse_info["str"];
   
$bytes_parsed = $bytes_parsed + strlen($str);

    return array(
$str, $bytes_parsed);
}

?>
up
-1
kobrasrealm at gmail dot com
2 years ago
I wrote a quick pair of functions using pack/unpack for converting between raw binary (e.g. openssl_random_pseudo_bytes() output) and hexadecimal (e.g. hash() output):

<?php
function raw2hex($raw) {
 
$m = unpack('H*', $raw);
  return
$m[1];
}

function
hex2raw($hex) {
  return
pack('H*', $hex);
}
?>

Feel free to suggest any improvements, but I thought this was worth sharing.
up
-1
Justin dot SpahrSummers at gmail dot com
8 years ago
I hadn't realized that if the number after the unpack type was 1 (i.e. "V1page"), that it would behave as if there was no number at all. I had been using a variable and didn't think to watch for this. For instance,

<?php

if ($something)
  
$get = 2;
else
  
$get = 1;

$arr = unpack("V" . $get . "page", $data);

?>

Now if $something was FALSE, then $arr will only have one entry named "page". If $something was TRUE, $arr would have "page1" and "page2".
up
-1
info at dreystone dot com
9 years ago
Here is my solution to reading a Big-Endian formatted double on an Little-Endian machine.

<?php

function ToDouble($data) {
   
$t = unpack("C*", pack("S*", 256));
    if(
$t[1] == 1) {
       
$a = unpack("d*", $data);
    } else {
       
$a = unpack("d*", strrev($data));
    }
    return (double)
$a[1];
}

?>
To Top