使用PHP将用户名发送到外部网页表单

I am logged into my website. From there I need to go to an external website and log in again with the same username/password. Instead of having to type in username/password again I want it to happen automatically. In the same way a browsers autofill does it when you visit a page you have been to before except I want to do it server side. I would like to do it in php.

This is the relevant section of the external webpage:

<div id="loginbox">
 <h3 id="login">Login</h3>
  <form action="" method="post" name="">
   <table class="login" border="0" cellspacing="1" cellpadding="3">
    <tbody>
     <tr>
      <td width="190" class="left">Username</td>
       <td width="695"><input name="username" type="text" id="username" size="30"></td>
     </tr>
    <tr>
     <td class="left">Password</td>
      <td><input name="password" type="password" id="password" size="30"></td>

    </tr>
   </tbody>
  </table>
 </div

Any assistance would be greatly appreciated.

A good option is create a RESTful Service API in PHP and your webform could comunicate with API to login and mutch more.

Some interesting tutorials: