Code:

function make_thumb($file_name,$thumb_file,$width,$height)
//$file_name - putanja do slike, $thumb_file smanjena slika sa putanjom, $width - sirina slike, $height - visina slike
{
    $sys=explode('.',$file_name);
    $br=count($sys)-1;
    if (preg_match(strtoupper('/jpg|jpeg/'),strtoupper($sys[$br])))
    {
        $src_img=imagecreatefromjpeg($file_name);
    }
    else
    {
        echo FILE_NOT_JPG; //Datoteka mora biti tipa jpg ili jpeg!
        exit;
    }
    $old_x=imageSX($src_img);
    $old_y=imageSY($src_img);
    if ($old_x>$old_y)
    {
        $thumb_w=$width;
        $thumb_h=$height;
    }
    else
    {
        $thumb_w=$height;
        $thumb_h=$width;
    }
    $dest_img=ImageCreateTrueColor($thumb_w,$thumb_h);
    imagecopyresampled($dest_img,$src_img,0,0,0,0,$thumb_w,$thumb_h,$old_x,$old_y);
    imagejpeg($dest_img,$thumb_file);
    imagedestroy($dest_img);
    imagedestroy($src_img);
}