title: Express Checkout error message: "Security header is not valid"
body: <p>I'm implementing <strong>Express Checkout</strong> in PayPal.</p>
  
  <p>I have no problem with the first two steps, <code>SetExpressCheckout</code> and <code>GetExpressCheckout</code>. But when I use  <code>DoExpressCheckout</code>, I encounter the error "Security header is not valid".</p>
  
  <p>The API credentials are the same!</p>
  
  <p>I've fixed it by changing the <code>$environment</code> to <code>live</code> in <code>DoExpressCheckout</code>. (The difference <code>$environment</code> makes is that it'll use <code>https://api.sandbox.paypal.com/nvp/</code> instead of <code>https://api-3t.$environment.paypal.com/nvp</code>)</p>
  
  <p>But why?</p>
  
  <p>Is there something wrong with <code>https://api-3t.$environment.paypal.com/nvp</code>?</p>