Machine name

/**
 * Convert human readable strings to machine names.
 *
 * @param string $str
 *   The string to convert.
 *
 * @return string
 *  The converted string.
 */
function machine_readable( $str ) {
	// $str must be a string.
	if ( ! is_string( $str ) ) {
		return '';
	}

	return preg_replace( '@[^a-z0-9_]+@', '_', strtolower( trim( $str ) ) );
}