Simple PHP formula

I have a form with two fields that lets user specify a start date and and end date. I'm using a date picker and uses the date format DD-MM-YYYY When this form is submitted, they are redirected to a thank-you page which is where I will need to place the script. This page will have the start and end dates as query strings in the URL. The URL is formatted like this (I'm just using example dates in the example below): [login to view URL] At the moment I can easily output the start and end dates on the thank-you page like this: echo "Thanks! Your start date is " . $_GET["startdate"] . "and your end date is " . $_GET["enddate"]; But I need to display more information on the thank-you page, and this is where you come in. 1) The total number of days For example, if the start date was 09-01-2016 and the end date was 11-01-2016, then on the thank-you page it should say "3 days". ...and... 2) The cost based on a formula The cost is $15 per day for the first 3 days, and $10 per day after that. So for example if the total number of days was 5, then the cost shown on the thank-you page would be $65.