You'll want to use the mb_convert_encoding() and htmlentities() functions, forcing UTF-8 conversion on incoming data, it's really the only safe way:
Code
<?php
error_reporting (E_ALL ^ E_NOTICE);
require('connect.php');
$name=$_POST['name'];
$name=mb_convert_encoding($name,'UTF-8', 'UTF-8');
$name=htmlentities($name, ENT_QUOTES, 'UTF-8');
$comment=$_POST['comment'];
$comment=mb_convert_encoding($comment,'UTF-8', 'UTF-8');
$comment=htmlentities($comment, ENT_QUOTES, 'UTF-8');
$submit=$_POST['submit'];
The submit var isn't in any real threat here, so I left it alone.