Should I keep variables used only in one class method local or declare them as class properties?

Question

I've been wondering if a class property is instantiated and used only in one class method should it be a class property at all or should it just be a local variable accessible to that class method only?

For example, should I keep a variable only used in one method as a local variable like this:

class myClass
{
    public function myMethod()
    {
        $_myVariableUsedOnlyOnce = "Hello World";
        echo $_myVariableUsedOnlyOnce;
    }
}

Or should I make the variable a private class property like this:

class myClass
{
    private $_myVariableUsedOnlyOnce;

    public function myMethod()
    {
        $this->_myVariableUsedOnlyOnce = "Hello World";
        echo $this->_myVariableUsedOnlyOnce;
    }
}

Which approach "smells"? What are the benefits to making all method variables class properties other than when I need to print_r() the entire object for debugging purposes?

Thanks

Answer 1

If you need it to have persistence across function calls, a class property would be best so that it moves around as the object does. You also might want to use it for other reasons in future in other functions. However, it does add overhead.

Generally, the class should have some real-world analogue, so if your variable corresponds to something that makes sense e.g. a person class has a $height, then it belongs as a class property. Otherwise, if it's just a part of the internal calculations of a method, then it doesn't really belong attached to the class e.g. a person does not have a $shoelaceIterator or whatever.

I'd argue that a confusing object design would be more of a smell than a potentially small memory overhead (although this depends on how big the variable is).

Answer 2

These local variables are not properties of your object.

They are not defining your object, then they should not be declared as private member.