Your question isn't a PHP problem It is a XHTML/HTML question.
The following works in Opera 7.54, Internet Explorer 6, Netscape 7.2, and Firefox 1.0.
Is this what you want?
<html>
<head>
<title>Simple center</title>
</head>
<body>
<table cellspacing="0" cellpadding="0" border="1" height="100%" width="100%">
<tr>
<td valign="middle" align="center">
Some text
</td>
</tr>
<table>
</body>
</html>