How to develop a completely secure php and python site?

Question

What is the requirement except of this:
1. use password hash
2. use bind param in database
3. mail or phone verification register

Answer 1

//Bugs free code

//Protect against XSS attacks

//Watch out for SQL injection

//Validate on both sides

//Check your passwords(change twice a month)

//Avoid file uploads

//Use HTTPS

--And there are many ways you can secure your sites