Online PHP editor | output for cJMtn

<?php const WEEK_YEAR_FORMAT = 'o-W'; function subtract_weeks_from_iso_week( string $start, int $weeks ): string { // Inputs are guaranteed (per usage) to be in 'o-W' => 'YYYY-WW' format. [ $start_year, $start_week ] = array_map( 'intval', explode( '-', $start ) ); $start_date = ( new DateTimeImmutable() )->setISODate( $start_year, $start_week ); $result_date = $start_date->modify( '-' . $weeks . ' weeks' ); return $result_date->format( WEEK_YEAR_FORMAT ); } echo subtract_weeks_from_iso_week(null, 4);