In index.php...
assuming that you can't center it in the actual include... why not center it using html?
<?php
// page - index.php
general_php_code();
?>
<div align=center>
<?php include('top10plr.html'); ?>
</div>
<?php
//eof
?>
Change your quotes from double quotes to single quotes for things that do not need to be parsed by PHP. SInce top10plr.html is not a variable, it doesn't need to be parsed. However, if your include was dynamic and used a variable, then you would need to use double quotes.
Anything contained within double quotes are parsed by PHP... By using single quotes, you can speed up your script. Examples where single quotes will suffice
$tbl_customers = 'customers';
$str_quote = 'My computer sucks';
Examples where you need double quotes:
$template = mysql_fetch_array("SELECT template FROM $tbl_templates WHERE id <=10 ORDER BY id");
<?php include("$template[1]"); ?>
Just an optimization tip I thought you might want from your example below.