OpenCart 🇺🇦

$this->language->load('checkout/checkout');

$json = array();		

// Validate if shipping is required. If not the customer should not have reached this page.
if (!$this->cart->hasShipping()) {
	$json['redirect'] = $this->url->link('checkout/checkout', '', 'SSL');
}

// Validate if shipping address has been set.		
$this->load->model('account/address');

if ($this->customer->isLogged() && isset($this->session->data['shipping_address_id'])) {					
	$shipping_address = $this->model_account_address->getAddress($this->session->data['shipping_address_id']);		
} elseif (isset($this->session->data['guest'])) {
	$shipping_address = $this->session->data['guest']['shipping'];
}

if (empty($shipping_address)) {								
	$json['redirect'] = $this->url->link('checkout/checkout', '', 'SSL');
}

// Validate cart has products and has stock.	
if ((!$this->cart->hasProducts() && empty($this->session->data['vouchers'])) || (!$this->cart->hasStock() && !$this->config->get('config_stock_checkout'))) {
	$json['redirect'] = $this->url->link('checkout/cart');				
}	

// Validate minimum quantity requirments.			
$products = $this->cart->getProducts();

foreach ($products as $product) {
	$product_total = 0;

	foreach ($products as $product_2) {
		if ($product_2['product_id'] == $product['product_id']) {
			$product_total += $product_2['quantity'];
		}
	}		

	if ($product['minimum'] > $product_total) {
		$json['redirect'] = $this->url->link('checkout/cart');

		break;
	}				
}

if (!$json) {
	if (!isset($this->request->post['shipping_method'])) {
		$json['error']['warning'] = $this->language->get('error_shipping');
	} else {
		$shipping = explode('.', $this->request->post['shipping_method']);

		if (!isset($shipping[0]) || !isset($shipping[1]) || !isset($this->session->data['shipping_methods'][$shipping[0]]['quote'][$shipping[1]])) {			
			$json['error']['warning'] = $this->language->get('error_shipping');
		}
	}

	if (!$json) {
		$shipping = explode('.', $this->request->post['shipping_method']);

		$this->session->data['shipping_method'] = $this->session->data['shipping_methods'][$shipping[0]]['quote'][$shipping[1]];

		$this->session->data['comment'] = strip_tags($this->request->post['comment']);
	}							
}

$this->response->setOutput(json_encode($json));