Adding Special Price to Variable Price

functions.php

<?php

/**
 * Adding Special Price to Variable Price.
 *
 * @param string $time_slot_price Booking Price.
 * @param string $product_id Product ID.
 * @param string $variation_id Variable ID.
 * @param string $product_type Product type.
 *
 * @return string $time_slot_price Modified Booking Price.
 */
function bkap_add_special_price_to_variation( $time_slot_price, $product_id, $variation_id, $product_type ) {

	if ( isset( $_POST['special_booking_price'] ) && '' != $_POST['special_booking_price'] ) {
		$time_slot_price = bkap_common::bkap_get_price( $product_id, $variation_id, $product_type );
		$time_slot_price +=	$_POST['special_booking_price'];
	}
	return $time_slot_price;
}
add_filter( 'bkap_modify_booking_price', 'bkap_add_special_price_to_variation', 10, 4 );