Redimensionar imágenes con php

Clase para redimensionar imágenes con php.

<?php
/**
 * Resize image class will allow you to resize an image
 *
 * Can resize to exact size
 * Max width size while keep aspect ratio
 * Max height size while keep aspect ratio
 * Automatic while keep aspect ratio
 */
class ResizeImage
{
private $ext;
private $image;
private $newImage;
private $origWidth;
private $origHeight;
private $resizeWidth;
private $resizeHeight;
/**
 * Class constructor requires to send through the image filename
 *
 * @param string $filename - Filename of the image you want to resize
 */
public function __construct( $filename )
{
if(file_exists($filename))
{
$this->setImage( $filename );
} else {
throw new Exception('Image ' . $filename . ' can not be found, try another image.');
}
}
/**
 * Set the image variable by using image create
 *
 * @param string $filename - The image filename
 */
private function setImage( $filename )
{
$size = getimagesize($filename);
$this->ext = $size['mime'];
switch($this->ext)
    {
    // Image is a JPG
        case 'image/jpg':
        case 'image/jpeg':
        // create a jpeg extension
            $this->image = imagecreatefromjpeg($filename);
            break;
        // Image is a GIF
        case 'image/gif':
            $this->image = @imagecreatefromgif($filename);
            break;
        // Image is a PNG
        case 'image/png':
            $this->image = @imagecreatefrompng($filename);
            break;
        // Mime type not found
        default:
            throw new Exception("File is not an image, please use another file type.", 1);
    }
    $this->origWidth = imagesx($this->image);
    $this->origHeight = imagesy($this->image);
}
/**
 * Save the image as the image type the original image was
 *
 * @param  String[type] $savePath     - The path to store the new image
 * @param  string $imageQuality   - The qulaity level of image to create
 *
 * @return Saves the image
 */
public function saveImage($savePath, $imageQuality="100", $download = false)
{
    switch($this->ext)
    {
        case 'image/jpg':
        case 'image/jpeg':
        // Check PHP supports this file type
            if (imagetypes() & IMG_JPG) {
                imagejpeg($this->newImage, $savePath, $imageQuality);
            }
            break;
        case 'image/gif':
        // Check PHP supports this file type
            if (imagetypes() & IMG_GIF) {
                imagegif($this->newImage, $savePath);
            }
            break;
        case 'image/png':
            $invertScaleQuality = 9 - round(($imageQuality/100) * 9);
            // Check PHP supports this file type
            if (imagetypes() & IMG_PNG) {
                imagepng($this->newImage, $savePath, $invertScaleQuality);
            }
            break;
    }
    if($download)
    {
    header('Content-Description: File Transfer');
header("Content-type: application/octet-stream");
header("Content-disposition: attachment; filename= ".$savePath."");
readfile($savePath);
    }
    imagedestroy($this->newImage);
}
/**
 * Resize the image to these set dimensions
 *
 * @param  int $width        - Max width of the image
 * @param  int $height       - Max height of the image
 * @param  string $resizeOption - Scale option for the image
 *
 * @return Save new image
 */
public function resizeTo( $width, $height, $resizeOption = 'default' )
{
switch(strtolower($resizeOption))
{
case 'exact':
$this->resizeWidth = $width;
$this->resizeHeight = $height;
break;
case 'maxwidth':
$this->resizeWidth  = $width;
$this->resizeHeight = $this->resizeHeightByWidth($width);
break;
case 'maxheight':
$this->resizeWidth  = $this->resizeWidthByHeight($height);
$this->resizeHeight = $height;
break;
default:
if($this->origWidth > $width || $this->origHeight > $height)
{
if ( $this->origWidth > $this->origHeight ) {
     $this->resizeHeight = $this->resizeHeightByWidth($width);
   = $width;
} else if( $this->origWidth < $this->origHeight ) {
$this->resizeWidth  = $this->resizeWidthByHeight($height);
$this->resizeHeight = $height;
}
} else {
            $this->resizeWidth = $width;
            $this->resizeHeight = $height;
        }
break;
}
$this->newImage = imagecreatetruecolor($this->resizeWidth, $this->resizeHeight);
    imagecopyresampled($this->newImage, $this->image, 0, 0, 0, 0, $this->resizeWidth, $this->resizeHeight, $this->origWidth, $this->origHeight);
}
/**
 * Get the resized height from the width keeping the aspect ratio
 *
 * @param  int $width - Max image width
 *
 * @return Height keeping aspect ratio
 */
private function resizeHeightByWidth($width)
{
return floor(($this->origHeight/$this->origWidth)*$width);
}
/**
 * Get the resized width from the height keeping the aspect ratio
 *
 * @param  int $height - Max image height
 *
 * @return Width keeping aspect ratio
 */
private function resizeWidthByHeight($height)
{
return floor(($this->origWidth/$this->origHeight)*$height);
}
}
?>

Para redimensionar una imagen a un tamaño concreto (no respetará las proporciones)

$resize = new ResizeImage('images/Be-Original.jpg');
$resize->resizeTo(100, 100, 'exact');
$resize->saveImage('images/be-original-exact.jpg');

Para redimensionar a un ancho determinado (mantiene proporciones)

$resize = new ResizeImage('images/Be-Original.jpg');
$resize->resizeTo(100, 100, 'maxWidth');
$resize->saveImage('images/be-original-maxWidth.jpg');

Para redimensionar a un alto determinado (mantiene proporciones)

$resize = new ResizeImage('images/Be-Original.jpg');
$resize->resizeTo(100, 100, 'maxHeight');
$resize->saveImage('images/be-original-maxHeight.jpg');

Para redimensionar a un determinado tamaño la parte más grande de la imagen (ancho o alto)

$resize = new ResizeImage('images/Be-Original.jpg');
$resize->resizeTo(100, 100);
$resize->saveImage('images/be-original-default.jpg');

Para descargar la imagen redimensionada

$resize = new ResizeImage('images/Be-Original.jpg');
$resize->resizeTo(100, 100, 'exact');
$resize->saveImage('images/be-original-exact.jpg', "100", true);