Going to buy a php website. How to be sure it's not backdoored ?

Hello !
I'm buying a website made with php. How can I just be sure that there's no backdoor ? I'm going to invest all my money on this project so I started getting paranoid about this possibility

You can just skim through the code if you are a php coder or grep the php codes for functions use to execute system commands and retreiving its output. Functions like system(), exec().

However if you don't know jack diddly about PHP sites or how back doors work in general, it might be a good idea to have the site pen tested as soon as you gain control. There are tons of cheap service options out there, or hell I am sure you could make a post here and find someone willing to help ya out

Good luck bud, PHP is fun