When should I declare variables in a PHP class?

I'm new to the OOP paradigm, so there's probably a simple explanation for this question...

Do you always need to declare public object-wide variables in a class? For example:

<?php

class TestClass
{
    var $declaredVar;

    function __construct()
    {
        $this->declaredVar = "I am a declared variable.";
        $this->undeclaredVar = "I wasn't declared, but I still work.";
    }

    function display()
    {
        echo $this->declaredVar . "<br />";
        echo $this->undeclaredVar;
        echo "<br /><br />"; 
    }
}

$test = new TestClass;
$test->display();

$test->declaredVar = "The declared variable was changed.";
$test->undeclaredVar = "The undeclared variable was changed.";

$test->display();

?>

In this code, even though $declaredVar is the only declared variable, $undeclaredVar is just as accessible and useable--it seems to act as if I had declared it as public.

If undeclared class variables are always accessible like that, what's the point of declaring them all up front?

Solution 1:

That variable isn't uninitialized, it's just undeclared.

Declaring variables in a class definition is a point of style for readability. Plus you can set accessibility (private or public).

Anyway, declaring variables explicitly has nothing to do with OOP, it's programming-language-specific. In Java you can't do that because variables must be declared explicitly.

Solution 2:

If you declare a member inside the class you can set its accessibility e.g

private $varname;

Solution 3:

You should always declare your member variables and specify their accessibility within your classes. I like to put this information at the end of the class after my functions.

You should define them as soon as you have enough information to do so. Possibly in the constructor or via setter functions.

It is important to do this because it makes life much easier for people working with your code. They don't have to guess where different properties are coming from or why they're there. Also, most (if not all) IDEs will not pick up on class variables unless you've declared them somewhere. Code completion/hints are one of the many benefits of IDEs and without declaring your variables, you will render that functionality useless.