Resize an Image (on the fly) & Keep its Aspect Ratio using PHP and GD

Posted on September 28, 2008, under PHP 

This is a PHP Class useful if you need to resize images keeping their aspect ratio, using the GD Library. The new height is calculated proportionally to the new width’s size and reverse. For instance you have an image with the following dimensions: width – 1000, height – 800. Its thumbnail with a width of 250 will have the height of 200 (the ratio is kept).

Here’s the class (I will explain you how it works below):


Credits: Bit Repository

Source URL:
class Resize_Image {

var $image_to_resize;
var $new_width;
var $new_height;
var $ratio;
var $new_image_name;
var $save_folder;

function resize()
  exit("File ".$this->image_to_resize." does not exist.");

$info = GetImageSize($this->image_to_resize);

  exit("The file ".$this->image_to_resize." doesn't seem to be an image.");

$width = $info[0];
$height = $info[1];
$mime = $info['mime'];

Keep Aspect Ratio?

Improved, thanks to Larry

// if preserving the ratio, only new width or new height
// is used in the computation. if both
// are set, use width

if (isset($this->new_width))
$factor = (float)$this->new_width / (float)$width;
$this->new_height = $factor * $height;
else if (isset($this->new_height))
$factor = (float)$this->new_height / (float)$height;
$this->new_width = $factor * $width;
exit(”neither new height or new width has been set”);

// What sort of image?

$type = substr(strrchr($mime, '/'), 1);

switch ($type)
case 'jpeg':
    $image_create_func = 'ImageCreateFromJPEG';
    $image_save_func = 'ImageJPEG';
	$new_image_ext = 'jpg';

case 'png':
    $image_create_func = 'ImageCreateFromPNG';
    $image_save_func = 'ImagePNG';
	$new_image_ext = 'png';

case 'bmp':
    $image_create_func = 'ImageCreateFromBMP';
    $image_save_func = 'ImageBMP';
	$new_image_ext = 'bmp';

case 'gif':
    $image_create_func = 'ImageCreateFromGIF';
    $image_save_func = 'ImageGIF';
	$new_image_ext = 'gif';

case 'vnd.wap.wbmp':
    $image_create_func = 'ImageCreateFromWBMP';
    $image_save_func = 'ImageWBMP';
	$new_image_ext = 'bmp';

case 'xbm':
    $image_create_func = 'ImageCreateFromXBM';
    $image_save_func = 'ImageXBM';
	$new_image_ext = 'xbm';

	$image_create_func = 'ImageCreateFromJPEG';
    $image_save_func = 'ImageJPEG';
	$new_image_ext = 'jpg';

	// New Image
	$image_c = ImageCreateTrueColor($this->new_width, $this->new_height);

	$new_image = $image_create_func($this->image_to_resize);

	ImageCopyResampled($image_c, $new_image, 0, 0, 0, 0, $this->new_width, $this->new_height, $width, $height);

	       $new_name = $this->new_image_name.'.'.$new_image_ext;
	       $new_name = $this->new_thumb_name( basename($this->image_to_resize) ).'_resized.'.$new_image_ext;

		$save_path = $this->save_folder.$new_name;
		/* Show the image without saving it to a folder */
		   header("Content-Type: ".$mime);


		   $save_path = '';

	    $process = $image_save_func($image_c, $save_path);

		return array('result' => $process, 'new_file_path' => $save_path);


	function new_thumb_name($filename)
	$string = trim($filename);
	$string = strtolower($string);
	$string = trim(ereg_replace("[^ A-Za-z0-9_]", " ", $string));
	$string = ereg_replace("[ tnr]+", "_", $string);
	$string = str_replace(" ", '_', $string);
	$string = ereg_replace("[ _]+", "_", $string);

	return $string;

Variables Info

$image_to_resize – Here we set the full path (i.e. /home/ to the image we wish to resize. If the image is in the same folder with the class, then you can just set the filename as the path (i.e. mywallpaper.jpg).

The $new_width & $new_height are the new dimensions for the resized image.

$ratio – Do you wish to keep the image’s ratio aspect after resize? If it’s set to true it will calculate the new dimensions based on the new width (if it’s bigger than the height) or the new height (if it’s bigger than width).

$new_image_name – It’s used to set a name for the new resized image. If it’s not set a name will be assigned automatically.

$save_folder – The path to the folder where the new image will be saved. Make sure it’s writable (0777). It no folder is set the new image will show in the browser without being saved.

Here’s an example of how you can use this class:

include 'resize.image.class.php';

$image = new Resize_Image;

$image->new_width = 200;
$image->new_height = 200;

$image->image_to_resize = "/home/"; // Full Path to the file

$image->ratio = true; // Keep Aspect Ratio?

// Name of the new image (optional) - If it's not set a new will be added automatically

$image->new_image_name = 'sunset_wallpaper_thumbnail';

/* Path where the new image should be saved. If it's not set the script will output the image without saving it */

$image->save_folder = 'thumbs/';

$process = $image->resize();

if($process['result'] && $image->save_folder)
echo 'The new image ('.$process['new_file_path'].') has been saved.';

How to invoke this class to generate thumbnails in a HTML page?

This class has an advantage: you can resize images ‘on the fly’ with it. This way you don’t have to necessarily save the resized image on the server. You can just output it. Here’s how you can call this class from a HTML page without using any PHP code in it:


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
  <title> Show Resized Image </title>
  <meta name="Author" content="Bit Repository">
  <meta name="Keywords" content="src, image, resize, on the fly">
  <meta name="Description" content="Display a resized image">
  <img src="resize_image.php?image=sunset.jpg&new_width=200&new_height=200">

As you can see, the resize_image.php script is called. Some info is needed to make the resize: original image (‘image’), a width (‘new_width’) and a height (‘new_height’). In many cases, if the ‘ratio aspect’ is enabled, you can specify only one measurement. The new dimensions will be calculated based on whether you choose: width or height. Here’s the resize_image.php script:



error_reporting(E_ALL ^ E_NOTICE);

include 'resize.image.class.php';

$resize_image = new Resize_Image;

// Folder where the (original) images are located with trailing slash at the end
$images_dir = 'images/';

// Image to resize
$image = $_GET['image'];

/* Some validation */
exit('The requested image does not exist.');

// Get the new with & height
$new_width = (int)$_GET['new_width'];
$new_height = (int)$_GET['new_height'];

$resize_image->new_width = $new_width;
$resize_image->new_height = $new_height;

$resize_image->image_to_resize = $images_dir.$image; // Full Path to the file

$resize_image->ratio = true; // Keep aspect ratio

$process = $resize_image->resize(); // Output image

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