title: Security header is not valid omnipay
body: <p>I 'm trying to implement paypal payment with laravel into my website.
  I used <code>omnipay</code> laravel  package and now <strong>I'm working with sandbox</strong> for testing.
  I verified 
  I configured my request like this:</p>
  
  <pre><code>$gateway = Omnipay::create('PayPal_Express');
  $gateway-&gt;setUsername('my_user_name_from_sandbox');
  $gateway-&gt;setPassword('my_passwrd_from_sandbox');
  $gateway-&gt;setSignature('my_signature_from_sandbox');
  
  $gateway-&gt;setTestMode(true);
  
  $response = $gateway-&gt;purchase(
  array(
   'cancelUrl' =&gt; 'http://localhost/payment_cancel',
    'returnUrl' =&gt; 'http://localhost/payment_success',
    'name'    =&gt; 'item11',
   'description' =&gt; 'description',
   'amount' =&gt; '50.00',
   'currency' =&gt; 'USD'
  )
  )-&gt;send();
  </code></pre>
  
  <p>and in the success function I copied the above code :</p>
  
  <pre><code>$response = $gateway-&gt;completePurchase(
                  array(
                      'cancelUrl' =&gt; 'http://localhost/payment_cancel',
                      'returnUrl' =&gt; 'http://localhost/payment_success', 
                      'amount' =&gt; '50.00',
                      'currency' =&gt; 'USD'
                  )
          )-&gt;send();
    $data = $response-&gt;getData(); // this is the raw response object
    echo '&lt;pre&gt;';
    print_r($data);
  </code></pre>
  
  <p>as a result I get failure transaction:</p>
  
  <pre><code>Array
  (
  [TIMESTAMP] =&gt; 2014-10-29T20:55:40Z
  [CORRELATIONID] =&gt; 34f24e6e8194c
  [ACK] =&gt; Failure
  [VERSION] =&gt; 85.0
  [BUILD] =&gt; 13565888
  [L_ERRORCODE0] =&gt; 10002
  [L_SHORTMESSAGE0] =&gt; Security error
  [L_LONGMESSAGE0] =&gt; Security header is not valid
  [L_SEVERITYCODE0] =&gt; Error
  )
  </code></pre>
  
  <p>I verified that the <strong>username,password and signature are the same as the sandbox parameters</strong>, and I verified that the returned <strong>url</strong> seperate parameters <strong>with <code>&amp;</code></strong>.</p>
  
  <p>--edit--</p>
  
  <p>I have read <a href="https://stackoverflow.com/questions/1712685/express-checkout-error-message-security-header-is-not-valid?lq=1">this</a>so that I checked the mentioned parameters in the omnipay package and I found this :</p>
  
  <pre><code>protected $liveEndpoint = 'https://api-3t.paypal.com/nvp';
  protected $testEndpoint = 'https://api-3t.sandbox.paypal.com/nvp';
  </code></pre>
  
  <p>the error is : <code>production.ERROR: exception 'Guzzle\Http\Exception\CurlException' with message '[curl] 6: Could not resolve host: api-3t.paypal.com</code>
  Could you please help me to resolve this problem?</p>