PHP: Quick and dirty JIT image resizing (i.e. thumbnails) with built-in caching

gistfile1.php

<?php

$storagePath = './img';
$cachePath = $storagePath.'/cache';

// Maximum requestable dimensions
$maxWidth = 1000;
$maxHeight = 1000;

$forceJpeg = true;
$defaultQuality = 100;

// Valid image types
$imageTypes = array(
    'jpg' => 'image/jpeg',
    'jpeg' => 'image/jpeg',
    'gif' => 'image/gif',
    'png' => 'image/png',
);

$upscale = !empty($_GET['upscale']);
$quality = !empty($_GET['quality']) ? ltrim(preg_replace('/[^\d]/', '', $_GET['quality']), '0') : $defaultQuality;
$width = !empty($_GET['width']) ? ltrim(preg_replace('/[^\d]/', '', $_GET['width']), '0') : $maxWidth;
$height = !empty($_GET['height']) ? ltrim(preg_replace('/[^\d]/', '', $_GET['height']), '0') : $maxHeight;
$imageName = !empty($_GET['image']) ? pathinfo($_GET['image'], PATHINFO_BASENAME) : '';
$extension = strtolower(pathinfo($imageName, PATHINFO_EXTENSION));
$imagePath = $storagePath.'/'.$imageName;

$quality = min($quality, 100);

if (
    !$width or
    !$height or
    !$imageName or
    !file_exists($imagePath) or
    !in_array($extension, array_keys($imageTypes))
)
{
    error_404();
}

$mime = $imageTypes[$extension];

$imagePath = $storagePath.'/'.$imageName;

if (!file_get_contents($imagePath)) error_404();

$funcSlug = ($extension == 'jpg') ? 'jpeg' : $extension;
$image = @call_user_func('imagecreatefrom'.$funcSlug, $imagePath);

if (!$image) error_404();

$currentWidth = imagesx($image);
$currentHeight = imagesy($image);

if (($currentWidth / $currentHeight) >= 1)
{
    $scale = min($currentWidth, $width, $maxWidth) / $currentWidth;
}
else
{
    $scale = min($currentHeight, $height, $maxHeight) / $currentHeight;
}

$width = $width * $scale;
$height = $height * $scale;

$hashedName = md5("{$imageName}@{$width}x{$height}");
$hashedPath = $cachePath.'/'.$hashedName;

if (file_exists($hashedPath))
{
    return_image($hashedPath, $mime);
}

// Don't want to scale images up unless requested
if ($scale >= 1 and !$upscale)
{
    return_image($imagePath, $mime);
}

$newWidth = $currentWidth * $scale;
$newHeight = $currentHeight * $scale;

$newImage = imagecreatetruecolor($newWidth, $newHeight);

imagecopyresampled(
    $newImage,
    $image,
    0, 0, 0, 0,
    $newWidth,
    $newHeight,
    $currentWidth,
    $currentHeight
);

imagedestroy($image);

if (!$newImage) error_404();

if ($forceJpeg)
{
    $funcSlug = $extension = 'jpeg';
    $mime = $imageTypes[$extension];
}

call_user_func('image'.$funcSlug, $newImage, $hashedPath, $quality);
return_image($hashedPath, $mime);

function error_404()
{
    header('HTTP/1.0 404 Not Found');
    echo 'Error';
    exit;
}

function return_image($image, $mime)
{
    header('Content-type: ' . $mime);
    echo file_get_contents($image);
    exit;
}