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-bottom: 10px; }
</style>
</head>
<body>
<center>
<?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
}
?>
</center>
</body>
</html>
yo creo que deves ponerle <center> como esta en este pequeño ejemplo que ise pruevalo