what does this code do?

$pp=4;

if (empty($_GET['page_number']))
{
        //default page/total = 1
        $pg=0;
        //get total num of pages
        $query_get_all="SELECT * FROM pictures order by id ASC";
        $res_get_all=mysql_query($query_get_all) or die(mysql_error());
        $get_all=mysql_num_rows($res_get_all);

        $pgtotal=$get_all;
        if ( $pgtotal%$pp==0) {
            $pgtotal/=$pp;
            $pgtotal--;
        }
        else {
            $pgtotal/=$pp; //$pgtotal++;
            $pgtotal=floor($pgtotal);
        }

}
else
{
        $pg=intval(strip_tags($_GET['page_number']))-1;
        //get total num of pages

        $query_get_all="SELECT * FROM pictures order by id ASC";
        $res_get_all=mysql_query($query_get_all) or die(mysql_error());
        $get_all=mysql_num_rows($res_get_all);

        $pgtotal=$get_all;
        if ( $pgtotal%$pp==0) {
            $pgtotal/=$pp;
            $pgtotal--;
        }
        else {
            $pgtotal/=$pp; //$pgtotal++;
            $pgtotal=floor($pgtotal);
        }
}

    $lim_inf=$pg*$pp;
    $lim_sup=$lim_inf+$pp;

    $select_picture = "SELECT * FROM pictures  ORDER BY id ASC LIMIT $lim_inf,$pp ";

    $result_select_picture=mysql_query($select_picture) or die(mysql_error());
    $noproducts = mysql_num_rows($result_select_picture);
?>