方法可以在类之外定义吗？

I'm just getting into using classes in php and am coming from a C++ background. Literally every example I've seen shows methods being defined inside of the class definition, and I'm wondering if I can define my class methods outside of the class definition in order to improve readability?

Sort of. PHP 5.4 itroduced traits which is about as close as you're gonna get.

In short, NO you can't do that; In php every mothod should be inside the class. And that is why php isn't considered to be fully OOP language by some developers i have seen.

Yes you can in PHP. The function body for class methods can be put fully into external files:

class foo
{
    function bar() {
        include('import_method_body.php');
    }
}

This works since ages.