Problem resizing the image in PHP - gd creates resized vague images


I am creating thumbnails of fixed height and width from my PHP script using the following function

/*creates thumbnail of required dimensions*/
function createThumbnailofSize($sourcefilepath,$destdir,$reqwidth,$reqheight,$aspectratio=false)
     * $sourcefilepath =  absolute source file path of jpeg
     * $destdir =  absolute path of destination directory of thumbnail ending with "/"
    $thumbWidth = $reqwidth; /*pixels*/
    $filename = split("[/\\]",$sourcefilepath);
    $filename = $filename[count($filename)-1];
    $thumbnail_path = $destdir.$filename;
    $image_file = $sourcefilepath;

    $img = imagecreatefromjpeg($image_file);
    $width = imagesx( $img );
    $height = imagesy( $img );

    // calculate thumbnail size
    $new_width = $thumbWidth;
        $new_height = floor( $height * ( $thumbWidth / $width ) );
        $new_height = $reqheight;

    // create a new temporary image
    $tmp_img = imagecreatetruecolor( $new_width, $new_height );

    // copy and resize old image into new image
    imagecopyresized( $tmp_img, $img, 0, 0, 0, 0, $new_width, $new_height, $width, $height );

    // save thumbnail into a file

    $returnvalue = imagejpeg($tmp_img,$thumbnail_path);
    return $returnvalue;

and I call this function with following parameters


but the problem is the resulting image is of very poor quality, when I perform the same operation with Adobe Photo shop, it performs a good conversion.. why it is so? I am unable to find any quality parameter, through which I change the quality of output image..

Use imagecopyresampled() instead of imagecopyresized().