Custom query with conditional taxonomy args

custom-query-conditional-args.php

<?php

	// Set up default query
	$locations_query_args = [
		'post_type'      => 'locations',
		'post_status'    => 'publish',
		'posts_per_page' => 500,
	];

	// Add the search query if set ($town_city_selected)
	if ( false !== $town_city_selected ) {
		$locations_query_args['s'] = esc_attr( $town_city_selected );
	}

	// Set the nearest station if set
	if ( false !== $nearest_station_selected ) {
		$locations_query_args['tax_query'][] = [
			'taxonomy' => 'nearest_station',
			'field'    => 'term_id',
			'terms'    => esc_attr( $nearest_station_selected ),
		];
	}

	// Set the theme(s) if set
	if ( false !== $themes_selected ) {
		$locations_query_args['tax_query'][] = [
			'taxonomy' => 'themes',
			'field'    => 'term_id',
			'terms'    => $themes_selected,
		];
	}

	// If BOTH taxonomies (nearest station and theme(s)) have a value then this is a relation = AND query (we need to deal with both)
	if ( false !== $nearest_station_selected && false !== $themes_selected ) {
		$locations_query_args['tax_query']['relation'] = 'AND';
	}

	// Process query
	$locations_query = new WP_Query( $locations_query_args );