A PHP "page" or a PHP "application"? If you need to write an application and you're thinking in terms of "page", you're missing something
Readable : Small, really small, smaller than that functions. Functions and objects that only do one thing, and just ONE thing. Good naming of functions/methods and variables. Your code should be readable like a small story. If you didn't already, read the book "Clean code". The code examples are in Java, but it's my favorite programming book of all time.
Efficient : Well, just avoiding the beginners pitfalls (like looping for queries, etc.) should do the trick here for the majority of an app.
After that, it's a matter of learning how to profile applications (database, CPU, code, etc.) and how to configure correctly the servers. Also, learning how to do performance tests helps a lot
Easy to refactore : If you write your code in OOP, learn about S.O.L.I.D.
Harder to introduce security holes : Well, it depends vastly on the people writing the code. You can't prevent people that don't know what they are doing to introduce security holes. But, if your code is already well done in terms of escaping data, SQL querying, etc... People that will add new features should, normally, use the same patterns.
Harder to create bugs : Mostly the same as the "security holes" topic.
If you're in a company with junior programmers, you need a senior to check their code (code review, all. the. time) or else they will introduce bugs and security holes. It's impossible for juniors to write super clean code without any bugs and security holes (and that's normal, they are just starting).
And, as other already said, using a framework will help in a couple of those points, but it's not the only thing you need (but you should definitely use one if you're in a company and want to code an application).
And maybe consider a templating system, it will separate HTML and PHP. The way you did it (PHP echoing HTML) is not necessarily bad because it's just a loop, but could lead people to think that it's ok to just put PHP code in the middle of HTML. And, IMO, a templating system will make it easier to read.