Trucating magento cart and then adding product to cart gives 500 error

Question

Here we have firstly truncate our magento cart,

$cart = Mage::getModel('checkout/cart');                
  $cart->truncate()->save(); // remove all active items in cart page
  $cart->init();


  Mage::getSingleton('checkout/session')->clear();

After this, we tried to add products

$cart = Mage::getSingleton('checkout/cart'); 
   $cart->init();
   $cart->addProduct($id,$qty);

Its giving 500 Internal Server Error

Note: Its only giving error when we apply adding product quantity to max.

Ex. available qty. 100 here in my case $qty=100

Editedcode :

<?php 
include 'app/Mage.php';
Mage::app();

Mage::getSingleton('core/session', array('name' => 'frontend'));
// Need for start the session
$cartHelper = Mage::helper('checkout/cart');
 $items = $cartHelper->getCart()->getItems();
 echo count($items);
 foreach($items as $item)
 {
  $itemId = $item->getItemId();
  $cartHelper->getCart()->removeItem($itemId)->save();
 }
try {
    $product_id = '7896'; // Replace id with your product id
    $qty = '99'; // max qty with your qty
    $product = Mage::getModel('catalog/product')->load($product_id);
    $cart = Mage::getModel('checkout/cart');
    $cart->init();
    $cart->addProduct($product, array('qty' => $qty));
    $cart->save();
    Mage::getSingleton('checkout/session')->setCartWasUpdated(true);

} catch (Exception $e) {
    echo $e->getMessage();
}

Note : Our code is working perfectly for first time only, rest of all time its giving this error

Kindly help

Answer 1

$session = Mage::getSingleton('checkout/session');
$session->getQuote()->delete();
$session->clear();

$cart = Mage::getModel('checkout/cart');
$cart->setQuote($session->getQuote());

$product = Mage::getModel('catalog/product') -> load($product_id);
if ($product) {
    if ($product -> isSaleable()) {
        $cart -> addProduct($product, array('qty' => $qty));
    }
}

// $session->setCartWasUpdated(true); // Not really necessary - Vicary
$cart->save();

Answer 2

Try this

UPDATES

require_once("app/Mage.php");    
Mage::app();

try {
    $session = Mage::getSingleton('checkout/session');
    $session->start();
    $session->clear();

    $cart  = Mage::getModel('checkout/cart');
    $cart->init(); 
    $cart->truncate();     

    $product_id = '2'; // Replace id with your product id
    $qty = '99'; // max qty with your qty
    $product = Mage::getModel('catalog/product')->load($product_id);    

    $cart->addProduct($product, array('qty' => $qty));
    $cart->save();

    $session->setCartWasUpdated(true);
    echo "product Added";

} catch (Exception $e) {
    echo "<pre>";
    print_r($e->getMessage());
}