Why does this popular author convert all images to .jpg?

The code below is modified from an O'Reilly Book - Learning PHP, MySQL, and JavaScript which can be found here

Why are all image types converted to .jpg?

Does .jpg offer the best quality/size ratio?

  public static function upload()
    {    
    $email=$_SESSION['email'];
    $path1="i8.jpg";
    $path2="z_p/$email.jpg";
    $path3="i9.jpg";     
    $path4="z_p/$email-1.jpg";     
    if(move_uploaded_file($_FILES['ufile']['tmp_name'], $path2))
      {
      $typeok=TRUE;
      switch($_FILES['ufile']['type'])
        {
        case "image/gif":   
          $src = imagecreatefromgif($path2); 
          break;
        case "image/jpeg":  
        case "image/pjpeg": 
          $src = imagecreatefromjpeg($path2); 
          break;
        case "image/png":   
          $src = imagecreatefrompng($path2); 
          break;
        default:            
          $typeok = FALSE; 
          break;
        }  
      if($typeok)
        {
        list($w, $h) = getimagesize($path2);

        $tw  = $w;
        $th  = $h;

        /*Run 1*/

        $max = 50;
        if($w > $h && $max < $w)
          {
          $th = $max / $w * $h; 
          $tw = $max;
          }  
        elseif ($h > $w && $max < $h)
          {
          $tw = $max / $h * $w; 
          $th = $max;
          } 
        elseif ($max < $w)
          {
          $tw = $th = $max;
          } 
        $dst = imagecreatetruecolor($tw, $th);
        imagecopyresampled($dst, $src, 0, 0, 0, 0, $tw, $th, $w, $h);           
        imagejpeg($dst, $path2);
        imagedestroy($dst); 

        /* Rune 2 */

        $max = 20;
        if($w > $h && $max < $w)    
          {
          $th = $max / $w * $h; 
          $tw = $max;
          }  
        els开发者_如何学编程eif ($h > $w && $max < $h)
          {
          $tw = $max / $h * $w;     
          $th = $max;
          } 
        elseif ($max < $w)
          {
          $tw = $th = $max;
          }
        $dst = imagecreatetruecolor($tw, $th);
        imagecopyresampled($dst, $src, 0, 0, 0, 0, $tw, $th, $w, $h);       
        imagejpeg($dst, $path4);
        imagedestroy($dst);
        imagedestroy($src);
        }
      }
    else
      {
      copy($path1, $path2);
      copy($path3, $path4);
      }
    }

There are lots of nasty surprises you can hide inside a jpeg file (or any of a number of image formats). By always recreating an image this way, you gain a certain amount of confidence that the image your server issues is sanitized.

JPEG doesn't always offer the best size/quality ratio, It depends on the image content, if it has many colors with many gradients, or is scenery picture, JPEG may be the best option, but for something like screenshots PNG will offer the best size/quality ratio.