This generates the thumbnail code, want it to be 600 width, and then the height proportionally how to get?

This generates the thumbnail code, want it to be 600 width, and then the height proportionally how to get it????
The code is on-line, the author notes that can be proportional!
But I set the height to 0, and it doesn't work. Can only be set fixed!
Now I want to make the width 600, the height is proportional. How do you get it???

 /**
* Generate thumbnail functions (supported image formats: GIF, JPEG, PNG, and BMP)
* @author Ruxing.li
* @param string $src source picture path
* @param int $width thumbnail width (equal to zoom only when height is specified)
* @param int $width thumbnail height (equal to zoom only when width is specified)
* @param string $filename save path (direct output to browser if not specified)
* @return BOOL
*/
function Mkthumbnail ($src, $width = null, $height = NULL, $filename = null) {
if (!isset ($width) &&!isset ($height))
return false;
if (Isset ($width) && $width <= 0)
return false;
if (Isset ($height) && $height <= 0)
return false;

$size = getimagesize ($SRC);
if (! $size)
return false;

List ($src _w, $src _h, $src _type) = $size;
$SRC _mime = $size [' MIME '];
Switch ($src _type) {
Case 1:
$img _type = ' gif ';
Break
Case 2:
$img _type = ' jpeg ';
Break
Case 3:
$img _type = ' png ';
Break
Case 15:
$img _type = ' wbmp ';
Break
Default:
return false;
}

if (!isset ($width))
$width = $src _w * ($height/$src _h);
if (!isset ($height))
$height = $src _h * ($width/$src _w);

$imagecreatefunc = ' Imagecreatefrom '. $img _type;
$src _img = $imagecreatefunc ($SRC);
$dest _img = Imagecreatetruecolor ($width, $height);
Imagecopyresampled ($dest _img, $src _img, 0, 0, 0, 0, $width, $height, $src _w, $src _h);

$imagefunc = ' image '. $img _type;
if ($filename) {
$imagefunc ($dest _img, $filename);
} else {
Header (' Content-type: '. $src _mime);
$imagefunc ($dest _img);
}
Imagedestroy ($src _img);
Imagedestroy ($dest _img);
return true;
}

Here to set the picture to 600x proportional how to get?

$result = Mkthumbnail (' 1.JPG ', ' 2.jpg ');
?>

------to solve the idea----------------------
$result = Mkthumbnail (' 1.JPG ', +, NULL, ' 2.jpg ');

Didn't he say it clearly?
* @param int $width thumbnail width (equal to zoom only when height is specified)
* @param int $width thumbnail height (equal to zoom only when width is specified)

