I created an object, it's named carwash. I created a session and assigned that object for session, if I enter quantity then and press buy button, I have result (example 6):

Your shopping cart contains 6 items.
but when I enter nothing, I get:

Your shopping cart contains items.
How should I do? Thank you! Here is all my code:

index.php
PHP Code:
<?php
    session_start
(); 
    
    require_once 
'carwash.php';
    if (isset(
$_SESSION['encoded_cartopass'])) {
        
// first let's get the variable from the session
        
$encoded_cartopass $_SESSION['encoded_cartopass'];
        
        
// now let's unpack it
        
$cartopass unserialize($encoded_cartopass);
        
        
// echo quantity
        
echo "Your shopping cart contains {$cartopass->getQuantity()} items. <br /><br />";
    }
    else {
        echo 
"Your shopping cart contains 0 items. <br /><br />";
    }
?>

<form action="process.php" method="post">
    Quantity: <input type="text" name="quantity" id="quantity"/>
    <input type="submit" value="Buy" name="submit" />
</form>
process.php
PHP Code:
<?php

    
require_once 'carwash.php';

    if (isset(
$_POST['submit'])) {
        
        if (!isset(
$_SESSION['encoded_cartopass'])) {
            
            
// construct and set quantity
            
$cartopass = new carwash();
            
$cartopass->setQuantity($_POST['quantity']);
            
            
// construct and encode session
            
session_register('encoded_cartopass');
            
$_SESSION['encoded_cartopass'] = serialize($cartopass);
        }
        else {
            
// if session is existing, decode it and
            // increment quantity
            
$encoded_cartopass $_SESSION['encoded_cartopass'];
            
            
$cartopass unserialize($encoded_cartopass);
            
            
$cartopass->incrementQuantity($_POST['quantity']);
            
            
// encode class and assign to session and
            // session is used pass to index.php
            
$_SESSION['encode_cartopass'] = serialize($cartopass);
        }
        
    }
    
    echo 
"<script>window.location.href='index.php'</script>";
?>
carwash.php
PHP Code:
<?php
    
class carwash {
        
        private 
$carmake;
        private 
$caryear;
        private 
$quantity;
        
        public function 
getCarmake() {
            return 
$this->carmake;
        }
        
        public function 
setCarmake($carmake) {
            
$this->carmake $carmake;
        }
        
        public function 
getCaryear() {
            return 
$this->caryear;
        }
        
        public function 
setCaryear($caryear) {
            
$this->caryear $caryear;
        }
        
        public function 
getQuantity() {
            return 
$this->quantity;
        }
        
        public function 
setQuantity($quantity) {
            
$this->quantity $quantity;
        }
        
        public function 
incrementQuantity($quantity '') {
            if (empty(
$quantity)) {
                
$this->quantity++;
            }
            else {
                
$this->quantity += $quantity;
            }
        }
        
        public function 
washcar() {
            echo 
"scruba, dub, dub, scruba, dub, dub <br />";
            echo 
"I'm feelling cleaner, Thank you!";
        }
    }
?>