title: paypal express checkout =>Error: Security header is not valid
body: <p>Error : Security header is not valid
  Array</p>
  
  <pre><code>(
      [TIMESTAMP] =&gt; 2014%2d04%2d29T07%3a24%3a29Z
      [CORRELATIONID] =&gt; 6af6749c848d6
      [ACK] =&gt; Failure
      [VERSION] =&gt; 109%2e0
      [BUILD] =&gt; 10800277
      [L_ERRORCODE0] =&gt; 10002
      [L_SHORTMESSAGE0] =&gt; Security%20error
      [L_LONGMESSAGE0] =&gt; Security%20header%20is%20not%20valid
      [L_SEVERITYCODE0] =&gt; Error
  )
  </code></pre>
  
  <p>I am getting this error again and again and cannot move ahead. I had checked all the api username, password and signature, all are correct, but the main problem is that, all the things are already stored in paypal sandbox account but it cannot give back to the page. I mean to say transection is shown in sandbox account but it cannot send forward to the display page. Am I wrong somewhere?</p>