PHP Detect PNG8 or PNG24

Question

I need an easy way to detect what PNG is being being uploaded. I tried getimagesize but it returns bits 8 for both PNG8 and PNG24.

Anyone have an idea how to detect it an image is PNG8, aka bit depth of 8. Thanks!

Answer 1

Try this to detect PNG8, PNG24 and PNG32:

<?php

$file = new SplFileObject('png8.png');

$realPath = $file->getRealPath();
if ($realPath === false) {
    throw new RuntimeException(sprintf('File not found: %s', $file->getFilename()));
}

$file->rewind();

if ($file->fread(4) !== chr(0x89) . 'PNG') {
    // This is not a PNG
    throw new RuntimeException(sprintf('Invalid PNG file: %s', $file->getFilename()));
}

$file->rewind();
$file->fread(8 + 4);
$idr = $file->fread(4);

// Make sure we have an IHDR
if ($idr !== 'IHDR') {
    throw new RuntimeException('No PNG IHDR header found, invalid PNG file.');
}

// PNG actually stores Width and height integers in big-endian.
$width = unpack('N', (string)$file->fread(4))[1];
$height = unpack('N', (string)$file->fread(4))[1];

// Bit depth: 1 byte
// Bit depth is a single-byte integer giving the number of bits per sample or
// per palette index (not per pixel).
//
// Valid values are 1, 2, 4, 8, and 16, although not all values are allowed for all color types.
$bitDepth = ord((string)$file->fread(1));

// Pixel format
// https://en.wikipedia.org/wiki/Portable_Network_Graphics#Pixel_format

// Color type is a single-byte integer that describes the interpretation of the image data.
// Color type codes represent sums of the following values:
// 1 (palette used), 2 (color used), and 4 (alpha channel used).
//
// Valid values are 0, 2, 3, 4, and 6.
$colorType = ord((string)$file->fread(1));

$colorTypes = [
    0 => 'Greyscale',
    2 => 'Truecolour',
    3 => 'Indexed-colour',
    4 => 'Greyscale with alpha',
    6 => 'Truecolour with alpha',
];

$colorTypeText = $colorTypes[$colorType];

$pngType = '?';
// If the bitdepth is 8 and the colortype is 3 (Indexed-colour) you have a PNG8
if ($bitDepth === 8 && $colorType === 3) {
    $pngType = 'PNG8';
}

// If the bitdepth is 8 and colortype is 2 (Truecolour) you have a PNG24.
if ($bitDepth === 8 && $colorType === 2) {
    $pngType = 'PNG24';
}

// If the bitdepth is 8 and colortype is 6 (Truecolour with alpha) you have a PNG32.
if ($bitDepth === 8 && $colorType === 6) {
    $pngType = 'PNG32';
}

echo sprintf('Width: %s, Height: %s, Bit-Depth: %s, Color-Type: %s (%s), Type: %s',
    $width,
    $height,
    $bitDepth,
    $colorType,
    $colorTypeText,
    $pngType
);

Output:

Width: 1500, Height: 500, Bit-Depth: 8, Color-Type: 3 (Indexed-colour), Type: PNG8