PHP缩放图片

function resize_image($filename, $tmpname, $xmax, $ymax)  
{  
    $ext = explode(".", $filename);  
    $ext = $ext[count($ext)-1];  

    if($ext == "jpg" || $ext == "jpeg")  
        $im = imagecreatefromjpeg($tmpname);  
    elseif($ext == "png")  
        $im = imagecreatefrompng($tmpname);  
    elseif($ext == "gif")  
        $im = imagecreatefromgif($tmpname);  

    $x = imagesx($im);  
    $y = imagesy($im);  

    if($x <= $xmax && $y <= $ymax)  
        return $im;  

    if($x >= $y) {  
        $newx = $xmax;  
        $newy = $newx * $y / $x;  
    }  
    else {  
        $newy = $ymax;  
        $newx = $x / $y * $newy;  
    }  

    $im2 = imagecreatetruecolor($newx, $newy);  
    imagecopyresized($im2, $im, 0, 0, 0, 0, floor($newx), floor($newy), $x, $y);  
    return $im2;   
}