dHash implementation in PHP based on http://blog.iconfinder.com/detecting-duplicate-images-using-python/

dhash.php

<?php

function dHash($file, $size = 8)
{
    $hash = '';
    list($w, $h, $t) = getimagesize($file);
    $im = imagecreatetruecolor($size + 1, $size);
    imagefilter($im, IMG_FILTER_GRAYSCALE);
    switch($t) {
        case 1:
            $oi = imagecreatefromgif($file);
            break;
        case 2:
            $oi = imagecreatefromjpeg($file);
            break;
        case 3:
            $oi = imagecreatefrompng($file);
            break;
    }
    imagecopyresampled($im, $oi, 0, 0, 0, 0, $size + 1, $size, $w, $h);
    imagedestroy($oi);
    for ($y = 0; $y < $size; $y++) {
        $val = 0;
        for ($x = 0; $x < $size; $x++) {
            $curr = imagecolorat($im, $x, $y);
            $next = imagecolorat($im, $x + 1, $y);
            $val += ($curr > $next) ? pow(2, ($x + ($y * $size)) % 8) : 0;
        }
        $hash .= str_pad(dechex($val), 2, 0, STR_PAD_LEFT);
    }
    imagedestroy($im);
    return $hash;
}

dhash_alt.php

<?php

function dHashAlt($file)
{
    $hash = '';
    $size = 8;
    list($w, $h, $t) = getimagesize($file);
    $im = imagecreatetruecolor($size + 1, $size);
    imagefilter($im, IMG_FILTER_GRAYSCALE);
    switch($t) {
        case 1:
            $oi = imagecreatefromgif($file);
            break;
        case 2:
            $oi = imagecreatefromjpeg($file);
            break;
        case 3:
            $oi = imagecreatefrompng($file);
            break;
    }
    imagecopyresampled($im, $oi, 0, 0, 0, 0, $size + 1, $size, $w, $h);
    imagedestroy($oi);
    for ($y = 0; $y < $size; $y++) {
        $val = '';
        for ($x = 0; $x < $size; $x++) {
            $curr = imagecolorat($im, $x, $y);
            $next = imagecolorat($im, $x + 1, $y);
            $val .= ($curr > $next) ? 1 : 0;
        }
        $hash .= str_pad(dechex(bindec($val)), 2, 0, STR_PAD_LEFT);
    }
    imagedestroy($im);
    return $hash;
}