Question

I’m using this to redirect to the home page if the post doesn’t exist:

if($_GET['id'] > mysql_num_rows($total_rows)){
    header("HTTP/1.1 301 Moved Permanently"); 
    header("Location: ".site_url."/"); 
    exit(); 
}

is this SEO friendly?

Answer 1

A 301 redirect is the wrong thing to do here. You should issue a 404 Not Found with a nice error page that explains the status to the user (including a link to your homepage or anywhere else) or similarly a 410 Gone if the content existed before. A 301 is incorrect here and may or may not be SEO friendly; a 4xx status is certainly more correct. Whether this status is issued through PHP or any other means is irrelevant.

Also see http://en.wikipedia.org/wiki/Http_status_codes