出来は悪いのですが、よく使うしよく修正するので、とりあえずここに置いておきます。

Image.php

<?php



class ImageLoadingFailedException extends ErrorException {}

class UnSupportedImageException extends ErrorException {}


/**
 * Simply image processing descriptor.
 * @see http://jp.php.net/manual/ja/ref.image.php
 * @author yuka2py
 */
class Image
{
	const RESIZE_POLICY_IN_BOUND = 'InBound';
	const RESIZE_POLICY_OUT_BOUND = 'OutBound';
	const RESIZE_POLICY_FIT = 'Fit';

	static private $_suppoted_type_and_extensions = array(
		IMAGETYPE_PNG => 'png',
		IMAGETYPE_JPEG => 'jpeg',
		IMAGETYPE_GIF => 'gif',
	);

	/**
	 * リサイズ後のサイズを計算して返す
	 * @return array array(width, height)
	 * @param integer $width 目標サイズ
	 * @param integer $height 目標サイズ
	 * @param integer $src_width ソースサイズ
	 * @param integer $src_height ソースサイズ
	 * @param string $resize_policy リサイズの方針
	 * @param boolean $expand 目標サイズよりソースサイズが小さい時、拡大するか否か
	 */
	static private function _calc_resize(
			$width, $height, 
			$src_width, $src_height, 
			$resize_policy, 
			$expand) {
		$sx = $width / $src_width;
		$sy = $height / $src_height;

		switch ($resize_policy) {
		case Image::RESIZE_POLICY_FIT:
			$sx = $width / $src_width;
			$sy = $height / $src_height;
			break;
		case Image::RESIZE_POLICY_IN_BOUND:
			$sx = $width / $src_width;
			$sy = $height / $src_height;
			$sx = $sy = min($sx, $sy);
			break;
		case Image::RESIZE_POLICY_OUT_BOUND:
			$sx = $width / $src_width;
			$sy = $height / $src_height;
			$sx = $sy = max($sx, $sy);
			break;
		default:
			throw ErrorException(
				'Unexpected resize policy: ' . $resize_policy);
		}

		$w = $src_width * $sx;
		$h = $src_height * $sy;

		if ( ! $expand) {
			switch ($resize_policy) {
			case Image::RESIZE_POLICY_FIT:
				$w = min($w, $src_width);
				$h = min($h, $src_height);
				break;
			case Image::RESIZE_POLICY_IN_BOUND:
			case Image::RESIZE_POLICY_OUT_BOUND:
				if ($src_width < $w or $src_height < $h) {
					$w = $src_width;
					$h = $src_height;
				}
				break;
			}
		}

		$w = round($w);
		$h = round($h);
		return array($w, $h);
	}


	/**
	 * Create image instance.
	 * @return Image
	 * @param mixed $src  file path or url path or resource.
	 */
	static public function create($src, $type=null) {
		return new Image($src, $type);
	}
	/**
	 * Load image, return resource
	 * @return array array($resource, $type);
	 * @param string $src file path or url.
	 */
	static public function load_resource($src) {
		list(,, $type) = getimagesize($src);
		switch ($type) {
			case IMAGETYPE_PNG:
				$resource = imagecreatefrompng($src);
				break;
			case IMAGETYPE_GIF:
				$resource = imagecreatefromgif($src);
				break;
			case IMAGETYPE_JPEG:
				$resource = imagecreatefromjpeg($src);
				break;
			default:
				throw new UnSupportedImageException(var_export($type, 1)." at $src");
		}
		return array($resource, $type);
	}
	/**
	 * Save resource to file.
	 * @param resource $resource
	 * @param string $type
	 * @param string $path sava file path. if specify null then flush to stream.
	 * 	 * @param integer $quality
	 */
	static public function flush_resource($resource, $type, $path=null, $quality=90) {
		switch ($type) {
			case IMAGETYPE_PNG:
				imagepng($resource, $path);
				break;
			case IMAGETYPE_GIF:
				imagegif($resource, $path);
				break;
			case IMAGETYPE_JPEG:
				imagejpeg($resource, $path, $quality);
				break;
			default:
				throw new UnSupportedImageException("$type @ $path");
		}
	}
	/**
	 * Resizing image file.
	 * @return void
	 * @param string $source  Image file path.
	 * @param integer $w  Destination width
	 * @param integer $h  Destination height.
	 * @param string $resize_policy[default=self::RESIZE_POLICY_IN_BOUND]  Risizing policy
	 * @param boolean $expand[default=false]  The original image is smaller than specified, whether or not to expand. Default is false means no expand.
	 * @param mixed $dst_image_type[default=false is same origin]  Destination file type. Default is same original file type.
	 * @param mixed $destination[default=false is same origin]  Destination file path. Default is same original file path. If original from url then flush to stream.
	 */
	public static function resize_image_file($source, 
			$w, $h,
			$resize_policy=Image::RESIZE_POLICY_IN_BOUND, 
			$expand=false,
			$dst_image_type=false,
			$destination=false) {
		list ($src_image, $src_image_type) = Image::load_resource($source);
		$sw = imagesx($src_image);
		$sh = imagesy($src_image);
		if (empty($dst_image_type)) {
			$dst_image_type = $src_image_type;
		}
		if ($destination===false) {
			$destination = file_exists($source) ? $source : null;
		}
		list ($w, $h) = Image::_calc_resize(
			$w, $h, $sw, $sh, 
			$resize_policy, $expand);
		
//		if ($sw !== $w or $sh === $h) {
			$dst_image = imageCreateTrueColor($w, $h);
			imageCopyResampled($dst_image, $src_image,0,0,0,0, $w, $h, $sw, $sh);
			Image::flush_resource($dst_image, $dst_image_type, $destination);
//		}
//		else if ($src_file_path !== $dst_file_path) {
//			copy($src_file_path, $dst_file_path);
//		}
	}

	/**
	 * 画像拡張子をサポートしているか？
	 * @return boolean
	 * @param  string $ext
	 */
	static public function is_supported_image_extension($ext) {
		return in_array(strtolower($ext), self::supported_image_extensions());
	}
	/**
	 * 画像タイプをサポートしているか？
	 * @return boolean
	 * @param  string $ext
	 */
	static public function is_supported_image_type($type) {
		return in_array($type, self::supported_image_types());
	}
	/**
	 * サポートする画像拡張子の配列で返す。
	 * @return array
	 */
	static public function supported_image_extensions() {
		return array_values(self::$_suppoted_type_and_extensions);
	}
	/**
	 * サポートする画像タイプを配列で返す。
	 * @return array
	 */
	static public function supported_image_types() {
		return array_keys(self::$_suppoted_type_and_extensions);
	}
	

	
	
	protected $_resource;
	protected $_type; //画像の想定タイプ
	protected $_filepath; //現在のファイルパス


	protected function __construct($resource, $type=null) {
		if (is_string($resource)) {
			$this->_filepath = $resource;
			list ($resource, $type) = Image::load_resource($resource);
		}
		$this->_resource = $resource;
		$this->_type = $type;
	}
	/**
	 * 画像の幅を返す。
	 */
	public function width() {
		return imagesx($this->_resource);
	}
	/**
	 * 画像の高さを返す。
	 */
	public function height() {
		return imagesy($this->_resource);
	}
	
	/**
	 * 画像の形式を返す。
	 */
	public function type($type=null) {
		if (func_num_args() > 0) {
			$this->_type = $type;
		}
		if (empty($this->_type)) {
			throw ErrorException('Image type not set yet.');
		}
		return $this->_type;
	}
	/**
	 * 画像のファイルパスを帰す。
	 */
	public function filepath($filepath=null) {
		if (func_num_args() > 0) {
			$this->_filepath = $filepath;
		}
		if (empty($this->_filepath)) {
			throw new ErrorException('File path not set yet.');
		}
		return $this->_filepath;
	}
	/**
	 * 画像タイプに該当する拡張子を返す。
	 * @return string
	 */
	public function extension() {
		return self::$_suppoted_type_and_extensions[$this->type()];
	}

	/**
	 * Resizing image.
	 * @return Image return myself.
	 * @param integer $w  Destination width
	 * @param integer $h  Destination height.
	 * @param string $resize_policy[optional=true]  Risizing policy
	 * @param boolean $expand[optional=false]  The original image is smaller than specified, whether or not to expand. Default is false means no expand.
	 */
	public function resize(
			$w, $h, 
			$resize_policy=self::RESIZE_POLICY_IN_BOUND, 
			$expand=false) {
		$sw = $this->width();
		$sh = $this->height();
		list ($w, $h) = Image::_calc_resize($w, $h, $sw, $sh, $resize_policy, $expand);
		if ($sw != $w or $sh != $h) {
			$new = imageCreateTrueColor($w, $h);
			imageCopyResampled($new, $this->_resource, 0, 0, 0, 0, $w, $h, $sw, $sh);
			$this->_resource = $new;
		}
		return $this;
	}

	/**
	 * Save to file.
	 * @param integer $type  Image type
	 * @param string $path  File path location
	 * @param integer $quality  Quality value for JPEG. Range of 0 to 100. Default is 90.
	 */
	public function flush($type, $path=null, $quality=90) {
		$path or $path = $this->filepath();
		$type or $type = $this->type();
		Image::flush_resource($this->_resource, $path, $type, $quality);
	}





}