Improve current form

I have created the following simple form, but I want the form to be improved and secured as well as easy to update "add and edit fields and remove ones I know basic PHP so...".

As you can see in the files attached, there is HTML, JS and PHP.
I want to get rid of everything "JavaScripts" and make the form function using only PHP.

1) Security is very important - so please make sure you take the right steps to make the form secure.

2) When the form is sent - the submit button text changes from "Send" to "Sent, Thank you." and then it is disabled adding a class to the button which is '.sent' - I already have this function but it is using JS which I want to get rid of and only use php.

3) I want to be easily able to format the email - I know how but just telling you so you bare in mind while developing.

Please make sure the code is tidy so I can read it and update it. Thanks.