Unpacking binary data in PHP

To set the stage we will start with a programming problem, this will keep the discussion anchored to a relevant context. The problem is this : We want to write a function that takes a image file as an argument and tells us whether the file is a GIF image; irrelevant with whatever the extension the file may have. We are not to use any GD library functions.

A GIF file header

With the requirement that we are not allowed to use any graphics functions, to solve the problem we need to get the relevant data from the GIF file itself. Unlike a HTML or XML or other text format files, a GIF file and most other image formats are stored in a binary format. Most binary files carry a header at the top of the file which provides the meta information regarding the particular file. We can use this information to find out the type of the file and other things, such as height an width in case of a GIF file. A typical raw GIF header is shown below, using a hex editor such as WinHex.

The detailed description of the header is given below.

Offset   Length   Contents
  0      3 bytes  "GIF"
  3      3 bytes  "87a" or "89a"
  6      2 bytes  <Logical Screen Width>
  8      2 bytes  <Logical Screen Height>
 10      1 byte   bit 0:    Global Color Table Flag (GCTF)
                  bit 1..3: Color Resolution
                  bit 4:    Sort Flag to Global Color Table
                  bit 5..7: Size of Global Color Table: 2^(1+n)
 11      1 byte   <Background Color Index>
 12      1 byte   <Pixel Aspect Ratio>
 13      ? bytes  <Global Color Table(0..255 x 3 bytes) if GCTF is one>
         ? bytes  <Blocks>
         1 bytes  <Trailer> (0x3b)

So to check if the image file is a valid GIF, we need to check the starting 3 bytes of the header, which have the ‘GIF’ marker, and the next 3 bytes, which give the version number; either ’87a’ or ’89a’. It is for tasks such as these that the unpack() function is indispensable. Before we look at the solution, we will take a quick look at the unpack() function itself.

Using the unpack() function

unpack() is the complement of pack() – it transforms binary data into an associative array based on the format specified. It is somewhat along the lines of sprintf, transforming string data according to some given format. These two functions allow us to read and write buffers of binary data according to a specified format string. This easily enables a programmer to exchange data with programs written in other languages or other formats. Take a small example.

$data = unpack('C*', 'codediesel');

This will print the following, decimal codes for ‘codediesel’ :

  1 => int 99
  2 => int 111
  3 => int 100
  4 => int 101
  5 => int 100
  6 => int 105
  7 => int 101
  8 => int 115
  9 => int 101
  10 => int 108

In the above example the first argument is the format string and the second the actual data. The format string specifies how the data argument should be parsed. In this example the first part of the format ‘C’, specifies that we should treat the first character of the data as a unsigned byte. The next part ‘*’, tells the function to apply the previously specified format code to all the remaining characters.

Although this may look confusing, the next section provides a concrete example.

Grabbing the header data

Below is the solution to our GIF problem using the unpack() function. The is_gif() function will return true if the given file is in a GIF format.

function is_gif($image_file)
    /* Open the image file in binary mode */
    if(!$fp = fopen ($image_file, 'rb')) return 0;
    /* Read 20 bytes from the top of the file */
    if(!$data = fread ($fp, 20)) return 0;
    /* Create a format specifier */
    $header_format = 'A6version';  # Get the first 6 bytes

    /* Unpack the header data */
    $header = unpack ($header_format, $data);
    $ver = $header['version'];
    return ($ver == 'GIF87a' || $ver == 'GIF89a')? true : false;
/* Run our example */
echo is_gif("aboutus.gif");

The important line to note is the format specifier. The ‘A6′ characters specifies that the unpack() function is to get the the first 6 bytes of the data and interpret it as a string. The retrieved data is then stored in an associate array with the key named ‘version’.

Another example is given below. This returns some additional header data of the GIF file, including the image width and height.

function get_gif_header($image_file)
    /* Open the image file in binary mode */
    if(!$fp = fopen ($image_file, 'rb')) return 0;
    /* Read 20 bytes from the top of the file */
    if(!$data = fread ($fp, 20)) return 0;
    /* Create a format specifier */
    $header_format = 
            'A6Version/' . # Get the first 6 bytes
            'C2Width/' .   # Get the next 2 bytes
            'C2Height/' .  # Get the next 2 bytes
            'C1Flag/' .    # Get the next 1 byte
            '@11/' .       # Jump to the 12th byte
            'C1Aspect';    # Get the next 1 byte

    /* Unpack the header data */
    $header = unpack ($header_format, $data);
    $ver = $header['Version'];
    if($ver == 'GIF87a' || $ver == 'GIF89a') {
        return $header;
    } else {
        return 0;
/* Run our example */

The above example will print the following when run.

    [Version] => GIF89a
    [Width1] => 97
    [Width2] => 0
    [Height1] => 33
    [Height2] => 0
    [Flag] => 247
    [Aspect] => 0

Below we will go into the details of how the format specifier works. I’ll split the format, giving the details for each character.

$header_format = 'A6Version/C2Width/C2Height/C1Flag/@11/C1Aspect';
A - Read a byte and interpret it as a string. 
    Number of bytes to read is given next
6 - Read a total of 6 bytes, starting from position 0
Version - Name of key in the associative array where data 
    retrieved by 'A6' is stored
/ - Start a new code format
C - Interpret the next data as an unsigned byte
2 - Read a total of 2 bytes
Width - Key in the associative array
/ - Start a new code format
C - Interpret the data as an unsigned byte
2 - Read a total of 2 bytes
Height- Key in the associative array
/ - Start a new code format
C - Interpret the data as an unsigned byte
1 - Read a total of 2 bytes
Flag - Key in the associative array
/ - Start a new code format
@ - Move to the byte offset specified by the following number.
      Remember that the first position in the binary string is 0. 
11 - Move to position 11
/ - Start a new code format
C - Interpret the data as an unsigned byte
1 - Read a total of 1 bytes
Aspect - Key in the associative array

More format options can be found here. Although I’ve only shown a small example, the pack/unpack is capable of much complex work than presented here.

This site is a digital habitat of Sameer Borate, a freelance web developer working in PHP, MySQL and WordPress. I also provide web scraping services, website design and development and integration of various Open Source API's. Contact me at metapix[at]gmail.com for any new project requirements and price quotes.

10 Responses



September 22nd, 2010 at 11:25 pm

Nice, some time ago i’ve seen unpack funciton in python, and it make me wonder how does it work. Now it is all clear, thanks.



September 23rd, 2010 at 4:04 am

Your first code block under ‘Using the unpack() function’ uses the first argument of ‘H*’, but in the description you describe this as ‘C’ (which is presumably what the parameter should read.


September 23rd, 2010 at 4:08 am

Thanks for the correction!



October 5th, 2010 at 9:49 am

Awesome Example ! Well done. Please provide more examples like this. I liked you GAPI examples as well.



October 15th, 2010 at 5:10 am

great sample of playing with binary data in PHP, thanks for sharing


C++ maniac

August 25th, 2012 at 3:30 am

I ran across this article while working on parsing an MP4 file without use of external libraries.

My platform is x86/x64 and is little endian. MP4 files are encoded in big endian.

So applying the unpack ‘plain vanilla’ gave unexpected results.

So I now check the endianess of the platform with:

function isLittleEndian() {
$testint = 0x00FF;
$p = pack( ‘S’, $testint );
return $testint === current( unpack( ‘v’, $p ));

And if the endianess is indeed little, I strrev the data read before calling unpack.

Perhaps this is a helpful addition for anyone trying to takes the GIF example to file formats that conflict with the platform endianess like mine did.



Omar Benazzouz

November 14th, 2012 at 9:06 am

many thanks, it was very useful for me



January 10th, 2013 at 10:54 am

Could you help me out if jpg



April 13th, 2014 at 10:41 am

Just great man….! Great post…! this: unpack(‘C*’, ‘codediesel’); was exactly what im looking for…!



October 28th, 2014 at 12:50 am

Great example. I think there may be error though—for the flag variable, you’re only reading 1 byte instead of 2 as noted “Read a total of 2 bytes”.

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