OOP，直接将类变量添加到类中，而不是之前定义它们

Question

My question is very simple, I saw this code in a class:

class controller_activity {
    function __construct($args) {
        //the variable template is not defined or what happen??
        $this->template = new Template('activity');     
    }
    .......

Usually I see the declaration of variables in a class like "public $template" or similar, but this case, is the same that if I define those variables as usual?, can I add variables in this way directly?, without defining them before?, is it recomendable?

Answer 1

is the same that if I define those variables as usual?,

No, the definition is missing. get_object_vars will show them, but get_class_vars won't.

can I add variables in this way directly?,

Yes, that is possible. They will be publicly visible.

without defining them before?,

Yes, without defining them before. They will be publicly visible.

is it recomendable?

That depends. Normally not, but there are exceptions to this rule.