Image similarity algorithm

HashBuilder.php

<?php
# Algorithm found here: http://www.hackerfactor.com/blog/index.php?/archives/432-Looks-Like-It.html
class HashBuilder
{
    private $buf;
    
    function __construct()
    {
        $this->buf = imagecreatetruecolor(8, 8);
    }

    function __destruct()
    {
        imagedestroy($this->buf);
    }

    public function hash($imageData) 
    {
        $img = imagecreatefromstring($imageData);
        imagecopyresampled($this->buf, $img, 0, 0, 0, 0, 8, 8, imagesx($img), imagesy($img));
        imagedestroy($img);
        imagefilter($this->buf, IMG_FILTER_GRAYSCALE);

        $colors = array();
        for ($i = 0; $i < 8; $i++) {
            for ($j = 0; $j < 8; $j++) {
                $colors[] = imagecolorat($this->buf, $i, $j) & 0xFF;
            }
        }

        $avg = array_sum($colors) / 64;

        $bin = '';
        foreach ($colors as $color) {
            $bin .= $color > $avg ? '1' : '0';
        }
        
        return base_convert($bin, 2, 16);
    }
}