a simple image hashing algorithm

imagehash.php

<?php
function imageHash($image_file) {
	// check if the file is existed and is readable 
	if(file_exists($image_file) && is_readable($image_file)) {
		return false;
	}

	// get image dimensions  
	list($width, $height) = getimagesize($image_file);

	// new dimensions
	$newwidth	= 100;
	$newheight	= 100;
	$img 		= imagecreatefromjpeg($image_file);
	$dst 		= imagecreatetruecolor($newwidth, $newheight);

	// resize image to 100x100
	imagecopyresampled($dst, $img, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);

	// converts the image into grayscale
	imagefilter($dst, IMG_FILTER_GRAYSCALE);
	$color_arr = [];

	// scan image from min x-coordinate and min y-coordinate to max x-coordinate and y-coordinate
	for ($width=5;$width<=imagesx($dst)-5;$width++) {
		for ($height=5;$height<=imagesy($dst)-5;$height++) {
			// get RGB color index
			$rgb = imagecolorat($dst, $width, $height);
			// convert value to integer
			$r = ($rgb >> 16) & 0xFF;
			$g = ($rgb >> 8) & 0xFF;
			$b = $rgb & 0xFF;
			// get the value of RGB color for all pixels and round the value to get the color index average.
			$rounded = round((($r+$g+$b)/3) / 10) * 10;
			// increase the rounded index by one each time
			$color_arr[$rounded] +=1; 
		}
	}

	// try to ignore abnormal values 
	foreach ($color_arr as $color => $count) {
		$count = round($count / 30) * 30;
		// ignore color index that count less than 40 or big than 1000
		if($count>40 && $count<1000) {
			$color_arr[$color]=$count;
		} else {
			unset($color_arr[$color]);
		}
	}
	// use md5 to create hash from the array 
	$hash = md5(implode('', array_keys($color_arr)).implode('', array_values($color_arr)));
	return $hash;
}
?>