ho creato una classe che permette di dividere in pagine i record di una query... magicamente funziona!

Codice PHP:
<?php
    
class dividixpag
    
{
        var 
$currentPag;
        var 
$totPages;
        var 
$totElements;
        
        function 
dividixpag()
            {
                
//
            
}
            
        function 
dividiPag($query$pag 0$xPag 5)
            {
                global 
$db;
                
                
$this->currentPag $pag;
                
$db->query($query);
                
$this->totElements $db->num_rows;
                
                if(
$this->currentPag == 0)
                    {
                        
$this->totPages = -1;
                        
$limit_da 0;
                        
$num $this->totElements;
                    }
                else
                    {
                        
$this->totPages ceil($this->totElements $xPag);
                        
$limit_da = ($this->currentPag*$xPag-($xPag-1))-1;
                        
$limit_a $this->currentPag*$xPag;
                
                        if(
$limit_a>$this->totElements)
                            
$num $this->totElements $limit_da;
                        else    
                            
$num$xPag;
                            
                    }
                
                return array(
$limit_da$num);
            }
            
        function 
showPages()
        {
            
$link="index2.php";
            
            
$qs=$_SERVER['QUERY_STRING'];
            
$qs=eregi_replace("^(&p=)[0-9]*$","&p=",$qs);
            if (!
ereg("^(p=)$"$qs))
                
$qs.="&p=";
                
            
$link.="?".$qs;
            
                
            if(
$this->currentPag!=1)
                {
                    
$prec $this->currentPag 1;
                    echo 
"<a href=\"".$link."1\" title=\"Pag n. 1\">"."Prima"."</a>-\n";
                    echo 
"<a href=\"".$link.$prec."\" title=\"Pag n. ".$prec."\">&lt;&lt;&lt;</a>-\n";
                }
            else
                echo 
"Prima"." - &lt;&lt;&lt; - ";
            
            for(
$x=1$x<=$this->totPages$x++)
                {
                    if(
$x==$this->currentPag)
                        echo 
$x;
                    else
                        echo 
"    <a href=\"".$link.$x."\" title=\"Pag n. $x\">$x</a> \n";
                    
                    echo 
" - ";
                }
                
            if(
$this->currentPag!=$this->totPages)
                {
                    
$succ $this->currentPag 1;
                    echo 
"    <a href=\"".$link.$succ."\" title=\"Pag n. ".$succ."\">&gt;&gt;&gt;</a>-\n";
                    echo 
"    <a href=\"".$link.$succ."\" title=\"Pag n. ".$this->totPages."\">"."Ultima"."</a>\n";
                }
            else
                echo 
"&gt;&gt;&gt; - "."Ultima";
        }
        
        
    }
?>
Nel programma istanzio la classe carrello
$carrello = new carrello();

e la classe per dividere
$dividi = new dividixpag();

nel metodo carrello::getCarrelli() ho messo
global $db; // Recupera la connessione
global $dividi; Recupera l'oggetto per dividere la query in pagine...

solo che mi da: Fatal error: Call to a member function on a non-object in /web/htdocs/www.milanogiovanni.net/home/lib/php/classes/carrelli/carrelli.php on line 67

perchè?