OpenCart 🇺🇦

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

$json = array();

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

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

if (empty($payment_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['payment_method'])) {
		$json['error']['warning'] = $this->language->get('error_payment');
	} elseif (!isset($this->session->data['payment_methods'][$this->request->post['payment_method']])) {
		$json['error']['warning'] = $this->language->get('error_payment');
	}	

	if ($this->config->get('config_checkout_id')) {
		$this->load->model('catalog/information');

		$information_info = $this->model_catalog_information->getInformation($this->config->get('config_checkout_id'));

		if ($information_info && !isset($this->request->post['agree'])) {
			$json['error']['warning'] = sprintf($this->language->get('error_agree'), $information_info['title']);
		}
	}

	if (!$json) {
		$this->session->data['payment_method'] = $this->session->data['payment_methods'][$this->request->post['payment_method']];

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

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