Calculate a perceptual hash of an image

dHash2.php

<?php

require __DIR__ . '/vendor/autoload.php';

use Jcupitt\Vips\Access;
use Jcupitt\Vips\Image;
use Jcupitt\Vips\Interpretation;

/**
 * Calculate a perceptual hash of an image.
 * Based on the dHash gradient method - see http://www.hackerfactor.com/blog/index.php?/archives/529-Kind-of-Like-That.html
 *
 * @param Image $image
 *
 * @return string
 */
function dHash(Image $image): string
{
    /** @var Image $thumbnailImage */
    $thumbnailImage = $image->thumbnail_image(9, [
        'height' => 8,
        'size' => 'force',
        'auto_rotate' => false,
        'linear' => false
    ])->colourspace(Interpretation::B_W);

    $dHashImage = $thumbnailImage->extract_band(0)->writeToMemory();

    // Calculate dHash
    $hash = 0;
    $bit = 1;
    for ($y = 0; $y < 8; $y++) {
        for ($x = 0; $x < 8; $x++) {
            $left = $dHashImage[($x * 8) + $y];
            $right = $dHashImage[($x * 8) + $y + 1];

            // Each hash bit is set based on whether the left pixel is brighter than the right pixel
            if ($left > $right) {
                $hash |= $bit;
            }

            // Prepare the next loop
            $bit <<= 1;
        }
    }

    return sprintf('%016x', $hash);
}

$image = Image::newFromFile('img_0076.jpg', ['access' => Access::SEQUENTIAL]);

// 446a3af634aecaec
echo dHash($image).PHP_EOL;