code to add custom value to variable product woocommerce

 // Add a custom field before single add to cart

add_action( 'woocommerce_before_add_to_cart_button', 'custom_product_price_field', 5 );
function custom_product_price_field(){
    echo '<div class="custom-text text">
    <h3>Rental</h3>
    <label>Start Date:</label>
    <input type="date" name="rental_date" value="" class="rental_date" />
    <label>Period Rental:</label>
    <select name="custom_price" class="custom_price">
        <option value="70" selected="selected">2 days</option>
        <option value="40">4 days</option> 
    </select>
    </div>';
}

// Get custom field value, calculate new item price, save it as custom cart item data
add_filter('woocommerce_add_cart_item_data', 'add_custom_field_data', 20, 2 );
function add_custom_field_data( $cart_item_data, $product_id ){

/* FOR VARIABLE PRODUCT

$variation_id = $_POST['variation_id'];

	if ($variation_id) {
		$variation = new WC_Product_Variation($variation_id);
		//$new_price = $custom_price = $variation->get_regular_price()+98;
		if ( isset($_POST['tabvalue']) && ! empty($_POST['tabvalue']) ){
			$base_price = str_replace("$","",get_post_meta( $variation_id, 'variationtesting_'.$_POST['tabvalue'].'_totalprice', true ));
			$run_size = get_post_meta( $variation_id, 'variationtesting_'.$_POST['tabvalue'].'_runsize', true );
			
			$custom_price = $variation->get_regular_price();
			$cart_item_data['custom_data']['new_price'] = $base_price + $custom_price;
			$cart_item_data['custom_data']['run_size'] = $run_size;
		}
		//echo wc_price($custom_price);
	}	

*/

    if ( isset($_POST['rental_date']) && ! empty($_POST['rental_date']) ){
        $cart_item_data['custom_data']['date'] = $_POST['rental_date'];
    }
    if ( isset($_POST['custom_price']) && ! empty($_POST['custom_price']) ){
        $product      = wc_get_product($product_id); // The WC_Product Object
        $base_price   = (float) $product->get_regular_price(); // Product reg price
        $custom_price = (float) sanitize_text_field( $_POST['custom_price'] );

        $cart_item_data['custom_data']['base_price'] = $base_price;
        $cart_item_data['custom_data']['new_price'] = $base_price/100 * $custom_price;
        $cart_item_data['custom_data']['rental'] = $custom_price;
    }
    if ( isset($cart_item_data['custom_data']['new_price']) || isset($cart_item_data['custom_data']['date']) ){
        $cart_item_data['custom_data']['unique_key'] = md5( microtime() . rand() ); // Make each item unique
    }
    return $cart_item_data;
}

// Set the new calculated cart item price
add_action( 'woocommerce_before_calculate_totals', 'extra_price_add_custom_price', 20, 1 );
function extra_price_add_custom_price( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;

    if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 )
        return;

    foreach ( $cart->get_cart() as $cart_item ) {
        if( isset($cart_item['custom_data']['new_price']) )
            $cart_item['data']->set_price( (float) $cart_item['custom_data']['new_price'] );
    }
}

// Display cart item custom price details
add_filter('woocommerce_cart_item_price', 'display_cart_items_custom_price_details', 20, 3 );
function display_cart_items_custom_price_details( $product_price, $cart_item, $cart_item_key ){
    if( isset($cart_item['custom_data']['base_price']) ) {
        $product        = $cart_item['data'];
        $base_price     = $cart_item['custom_data']['base_price'];
        $product_price  = wc_price( wc_get_price_to_display( $product, array( 'price' => $base_price ) ) ) . '<br>';
        if( isset($cart_item['custom_data']['rental']) ) {
            $product_price .= $cart_item['custom_data']['rental'] == '70' ? __("2 days") : __("4 days");
        }
    }
    return $product_price;
}

// Display in cart item the selected date
add_filter( 'woocommerce_get_item_data', 'display_custom_item_data', 10, 2 );
function display_custom_item_data( $cart_item_data, $cart_item ) {
    if ( isset( $cart_item['custom_data']['date'] ) ){

        $cart_item_data[] = array(
            'name' => __("Chosen date", "woocommerce" ),
            'value' =>   date('d.m.Y', strtotime($cart_item['custom_data']['date'])),
        );
    }
    return $cart_item_data;
}