Source

File: wp-admin/includes/file.php

function get_home_path() {
	$home    = set_url_scheme( get_option( 'home' ), 'http' );
	$siteurl = set_url_scheme( get_option( 'siteurl' ), 'http' );

	if ( ! empty( $home ) && 0 !== strcasecmp( $home, $siteurl ) ) {
		$wp_path_rel_to_home = str_ireplace( $home, '', $siteurl ); /* $siteurl - $home */
		$pos                 = strripos( str_replace( '\\', '/', $_SERVER['SCRIPT_FILENAME'] ), trailingslashit( $wp_path_rel_to_home ) );
		$home_path           = substr( $_SERVER['SCRIPT_FILENAME'], 0, $pos );
		$home_path           = trailingslashit( $home_path );
	} else {
		$home_path = ABSPATH;
	}

	return str_replace( '\\', '/', $home_path );
}