Allowing others to post comments via PHP

[Kasracer]

I would like to add posting of comments a feature of my website. The problem is, I am worried I won't secure it correctly since I'm still new to PHP.

I tried a similiar system hosted on a different database to see how it would work.

This is the function calling:

add_comment(basic_formatter($_POST['id']), basic_formatter($_POST['name']), $_SERVER['REMOTE_ADDR'], basic_formatter($_POST['message'])); 

I use a function that formats the input:

function basic_formatter($message)
{
    $message = str_replace(">", ">", $message);
    $message = str_replace("<", "&lt;", $message);
    $message = str_replace("\"", "&quot;", $message);
    return nl2br($message);
} 

and here is the part where we add our stuff to the database:

function add_comment($post_id, $name, $ip, $message)
{
    connect();
    $result = mysql_query("INSERT INTO comments (post_id, name, ip, message) VALUES('$post_id', '$name', '$ip', '$message')");
    if (!$result) {
        die('Invalid query: ' . mysql_error());}
    header("Location: index.php?id=".$post_id);
} 

What do you think? Am I careful enough or am I lacking in security?

EDIT: I probably should have put this into the PHP forum. Sorry. Could someone please move it?