$obj = new img_opt();
$obj->max_width(180);
$obj->max_height(180);
$obj->image_path('./pfad/zum/orginal/Bild.jpg');
$obj->image_resize();
$obj->save('./pfad/zum/vorschau/bild.jpg');

class img_opt
{

 var $max_width;
 var $max_height;
 var $path;
 var $img;
 var $new_width;
 var $new_height;
 var $mime;
 var $image;
 var $width;
 var $height;
 var $cropBlank = false;

    function max_width($width)
    {
        $this->max_width = $width;
    }
    function max_height($height)
    {
        $this->max_height = $height;
    }
    function image_path($path)
    {
        $this->path = $path;
    }
    function get_mime()
    {
        $img_data = getimagesize($this->path);
        $this->mime = $img_data['mime'];
    }
    function create_image()
    {
        switch($this->mime)
        {
            case 'image/jpeg':
                $this->image = imagecreatefromjpeg($this->path);
            break;

            case 'image/gif':
                $this->image = imagecreatefromgif($this->path);
            break;

            case 'image/png':
                $this->image = imagecreatefrompng($this->path);
            break;
        }
    }
   function image_resize()
   {
           $this->get_mime();
           $this->create_image();
           $this->width = imagesx($this->image);
           $this->height = imagesy($this->image);
           $this->set_dimension();
           $image_resized = imagecreatetruecolor($this->new_width,$this->new_height);
           imagecopyresampled($image_resized, $this->image, 0, 0, 0, 0, $this->new_width, $this->new_height,$this->width, $this->height);
           #imagejpeg($image_resized,$this->path);
           $this->image = $image_resized;
           return $image_resized;
   }


 function set_dimension()
 {
  $w = $this->max_width;
  $h = $this->max_height;

  $sw = $this->width;
  $sh = $this->height;


  if (isset ($w) AND !isset ($h)) {
   // autocompute height if only width is set
   if($sw <= $w){
    $w = $sw;
   }

   $h = (100 / ($sw / $w)) * .01;
   $h = @round ($sh * $h);

  } elseif (isset ($h) AND !isset ($w)) {
   // autocompute width if only height is set
   if($sh <= $h){
    $h = $sh;
   }

   $w = (100 / ($sh / $h)) * .01;
   $w = @round ($sw * $w);
  
  } elseif (isset ($h) AND isset ($w)) {
   // get the smaller resulting image dimension if both height
   // and width are set and $constrain is also set
   $hx = (100 / ($sw / $w)) * .01;
   $hx = @round ($sh * $hx);

   $wx = (100 / ($sh / $h)) * .01;
   $wx = @round ($sw * $wx);

   if ($hx < $h) {
    $h = (100 / ($sw / $w)) * .01;
    $h = @round ($sh * $h);
   } else {
    $w = (100 / ($sh / $h)) * .01;
    $w = @round ($sw * $w);
   }


  }

  $this->new_height = $h;
  $this->new_width = $w;


 }

   function save($filename, $image_type=IMAGETYPE_JPEG, $permissions=null) {
      if( $image_type == IMAGETYPE_JPEG ) {
         imagejpeg($this->image,$filename,100);
      } elseif( $image_type == IMAGETYPE_GIF ) {
         imagegif($this->image,$filename);
      } elseif( $image_type == IMAGETYPE_PNG ) {
         imagepng($this->image,$filename,100);
      }
      if( $permissions != null) {
         chmod($filename,$permissions);
      }
   }

}