Creates and verifies full 16,777,216-color PNGs.

16777216.php

<?php
	ini_set("memory_limit", "256M");

	if (isset($argv[1]))
	{
		echo "Verifying image...\n";
		verify16777216($argv[1]);
	}
	else
	{
		echo "No path set.  Creating image...\n";
		create16777216();
	}

	function pixelToRGB($x, $y, $w)
	{
		$pixelNum = ($y * $w) + $x;

		$r = (int)($pixelNum % 256);
		$g = (int)(floor($pixelNum / 256) % 256);
		$b = (int)(floor($pixelNum / (256 * 256)));

		return array($r, $g, $b);
	}

	function create16777216()
	{
		$size = 4096;
		$img = imagecreatetruecolor($size, $size);

		for ($i = 0; $i < $size * $size; $i++)
		{
			$x = $i % $size;
			$y = floor($i / $size);

			$rgb = pixelToRGB($x, $y, $size);
			$color = imagecolorallocate($img, $rgb[0], $rgb[1], $rgb[2]);

			imagesetpixel($img, $x, $y, $color);
		}

		echo "Saving 16777216.png to current directory...\n";
		imagepng($img, "16777216.png", 0);
	}

	function verify16777216($file)
	{
		$size = 4096;
		$img = imagecreatefrompng($file);

		for ($i = 0; $i < $size * $size; $i++)
		{
			$x = $i % $size;
			$y = floor($i / $size);

			$theirRGB = imagecolorat($img, $x, $y);
			$theirR = ($theirRGB >> 16) & 0xFF;
			$theirG = ($theirRGB >> 8) & 0xFF;
			$theirB = $theirRGB & 0xFF;

			$ourRGB = pixelToRGB($x, $y, $size);

			if ($theirR !== $ourRGB[0] || $theirG !== $ourRGB[1] || $theirB !== $ourRGB[2])
			{
				die(
					"Images differ at x:" . $x . " y:" . $y . ".\n" .
					"Us:     r:" . $ourRGB[0] . " g:" . $ourRGB[1] . " b:" . $ourRGB[2] . "\n" .
					"Them:   r:" . $theirR    . " g:" . $theirG    . " b:" . $theirB . "\n"
				);
			}
		}

		exit("The images are identical.\n");
	}
?>