path_is_absolute( string $path )

function path_is_absolute( $path ) {
	/*
	 * This is definitive if true but fails if $path does not exist or contains
	 * a symbolic link.
	 */
	if ( realpath($path) == $path )
		return true;

	if ( strlen($path) == 0 || $path[0] == '.' )
		return false;

	// Windows allows absolute paths like this.
	if ( preg_match('#^[a-zA-Z]:\\\\#', $path) )
		return true;

	// A path starting with / or \ is absolute; anything else is relative.
	return ( $path[0] == '/' || $path[0] == '\\' );
}