Código PHP:
  
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style>
body { margin-top: 200px; margin-right; 10px; margin-bottom: 10px; margin-left: 340px }
</style>
</head>
<body>
<?php 
$password = "123456";
if ($_POST['password'] != $password) { 
?>
<table width="200" border="1" cellpadding="0">
  <tr>
    <th scope="col">Login password </th>
  </tr>
  <tr>
    <td><form id="form1" name="form1" method="post" action="">
      <label></label>
      <div align="center">
        <label>
        <input type="password" name="password" />
        </label>
      </div>
    </form>
    </td>
  </tr>
</table>
<?php 
}else{
?>
<table width="200" border="1" cellpadding="0">
  <tr>
    <th scope="col">1</th>
    <th scope="col">2</th>
    <th scope="col">3</th>
    <th scope="col">4</th>
  </tr>
</table>
<?php 
} 
?>
</body>
</html>    
 
 problemas con centrado
 problemas con centrado 



