PHP SSL redirection

is_ssl.php

<?php 
function is_ssl() {
  if ( isset($_SERVER['HTTPS']) ) {
		if ( 'on' == strtolower($_SERVER['HTTPS']) )
			return true;
		if ( '1' == $_SERVER['HTTPS'] )
			return true;
	} elseif ( isset($_SERVER['SERVER_PORT']) && ( '443' == $_SERVER['SERVER_PORT'] ) ) {
		return true;
	}
	if ( isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && strtolower($_SERVER['HTTP_X_FORWARDED_PROTO']) == 'https' ) {
		return true;
	}
	return false;
}

use like this :

    if( !is_ssl() ){
    $redirect= "https://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'];
    header("Location:$redirect");
    }

Just curious, what is the advantage of using this over redirecting the whole site to HTTPS?

You can use this on the whole site to https but the advantage is that it's easier to configure than htaccess, if someone visit the website without https, he will be redirected to the https version, you don't need to hardcode the domain name in the htaccess so it's also easier for locahost testing.