Function get_home_path() from wp-admin/includes/file.php

function get_home_path() {
	$home = get_option( 'home' );
	$siteurl = get_option( 'siteurl' );
	if ( $home != '' && $home != $siteurl ) {
	        $wp_path_rel_to_home = str_replace($home, '', $siteurl); /* $siteurl - $home */
	        $pos = strpos($_SERVER["SCRIPT_FILENAME"], $wp_path_rel_to_home);
			
	        $home_path = substr($_SERVER["SCRIPT_FILENAME"], 0, $pos);
			echo "HOME_PATH0 = $home_path;<br />";
		$home_path = trailingslashit( $home_path );
		echo "HOME_PATH1 = $home_path;<br />WPPATHREL2H: $wp_path_rel_to_home<br />POS: $pos<br /><br />";
	} else {
		$home_path = ABSPATH;
		echo "HOME_PATH2 = $home_path; <br />ABS: ".ABSPATH."<br />";
	}
	echo "HOME: $home<br />SITEURL: $siteurl<br />HOME_PATH3: $home_path<br />";
//exit;
	return $home_path;
}

Output when WordPress address (URL) and Blog address (URL) are changed at teh same time to a different value (both the same).

HOME_PATH0 = ;
HOME_PATH1 = /;
WPPATHREL2H: http://site.com
POS:

HOME: http://127.0.0.5
SITEURL: http://site.com
HOME_PATH3: /
HOME_PATH0 = ;
HOME_PATH1 = /;
WPPATHREL2H: http://site.com
POS:

HOME: http://127.0.0.5
SITEURL: http://site.com
HOME_PATH3: /
HOME_PATH2 = C:\websites\site/;
ABS: C:\websites\site/
HOME: http://site.com
SITEURL: http://site.com
HOME_PATH3: C:\websites\site/
HOME_PATH2 = C:\websites\site/;
ABS: C:\websites\site/
HOME: http://site.com
SITEURL: http://site.com
HOME_PATH3: C:\websites\site/