Can I programmatically determine if a PNG is animated?

APNG images are designed to be "camouflaged" as PNG for readers that not support them. That is, if a reader does not support them, it will just assume it is a normal PNG file and display only the first frame. That means that they have the same MIME type as PNG (image/png), they have the same magic number (89 50 4e 47 0d 0a 1a 0a) and generally they're saved with the same extension (although that is not really a good way to check for a file type).

So, how do you distinguish them?APNG have a "acTL" chunk in them. So, if you search for the string acTL (or, in hex, 61 63 54 4C (the 4 bytes before the chunk marker (i.e. 00 00 00 08) are the size of the chunk in big endian format, without counting the size, marker, or CRC32 at the end of the field)) you should be pretty good. To get it even better, check that this chunk appears before the first occurrence of the "IDAT" chunk (just look for IDAT).

This code (taken from http://foone.org/apng/identify_apng.php ) will do the trick:

<?php# Identifies APNGs# Written by Coda, functionified by Foone/Popcorn Mariachi#!9i78bPeIxI# This code is in the public domain# identify_apng returns:# true if the file is an APNG# false if it is any other sort of file (it is not checked for PNG validity)# takes on argument, a filename.function identify_apng($filename)    {    $img_bytes = file_get_contents($filename);    if ($img_bytes)        {        if(strpos(substr($img_bytes, 0, strpos($img_bytes, 'IDAT')),                  'acTL')!==false)            {        return true;        }        }    return false;    }?>

AFAIK, libraries that do not support APNG will just take the first frame of the PNG. In your case, you could just create a new image from the APNG (or PNG, JPEG, etc.) and re-save it as PNG. It should strip the animation data if using GD, unless the library's been updated to support APNG.

I'd like to suggest a more optimised version, which doesn't read the whole file, as those could be quite big, and still rely on the acTL before IDAT rule:

function identify_apng($filepath) {    $apng = false;    $fh = fopen($filepath, 'r');    $previousdata = '';    while (!feof($fh)) {        $data = fread($fh, 1024);        if (strpos($data, 'acTL') !== false) {            $apng = true;            break;        } elseif (strpos($previousdata.$data, 'acTL') !== false) {            $apng = true;            break;        } elseif (strpos($data, 'IDAT') !== false) {            break;        } elseif (strpos($previousdata.$data, 'IDAT') !== false) {            break;        }        $previousdata = $data;    }    fclose($fh);    return $apng;}

Speed is enhanced from 5x to 10x or more depending on how big the file is, and it also uses a lot less memory.

NB: this maybe could be tweaked more with the size given to fread or with the concatenation of the previous chunk with the current one. By the way, we need this concatenation as the acTL/IDAT word might be split between two read chunks.