Mike from Pornlaw once passed on to me what his father said -- If you want a guarantee buy a toaster. What makes you think you are worth the trouble of putting in a back door -- do you have a server with root access?
What do you want coded exactly?
How do we know you are going to pay :P
There are people that are honest and will do what they are paid for but it is your responsibility to do your due diligence and check your coders, admins or developers reputations and their work quality out. Personally, I will only pay an advance of 1/3 or a max of 1/2 for anything. Who are you and what is your reputation -- it works both ways in business.
If you want to learn basic HTML and PHP
https://www.w3schools.com/ maybe you can do some of the work yourself? If you are not into the technical end and into the marketing aspects -- pay to get the coding done.