get_website_year_start.php

/**
 * In:     . Out: 2018
 * In: 2018. Out: 2018
 * In: 2013. Out: 2013-18
 * In: 1998. Out: 1998-2018
 *
 * @param string $year
 * @return string
 */
function get_website_year_start( $year ) {
    switch( true ) {
        case empty( $year ): //no year
        case intval( $year ) == intval( date( 'Y' ) ): //same year
            $html = date( 'Y' );
            break;

        case substr( strval( $year ), 0, 2 ) === substr( strval( date( 'Y' ) ), 0, 2 ): //same century
            $html = $year . '-' . substr( strval( date( 'Y' ) ), 2, 2 );
            break;
        //diff years
        default:
            $html = $year . '-' . date( 'Y' );
    }

    return $html;
}