Is a property default value of null the same as no default value?

Question

And can I therefore safely refactor all instances of

class Blah
{
    // ...
    private $foo = null;
    // ...
}

to

class Blah
{
    // ...
    private $foo;
    // ...
}

?

Answer 1

Untyped properties

Simple answer, yes. See http://php.net/manual/en/language.types.null.php

The special NULL value represents a variable with no value. NULL is the only possible value of type null.

You can easily test by performing a var_dump() on the property and you will see both instances it will be NULL

class Blah1
{
    private $foo;

    function test()
    {
        var_dump($this->foo);
    }
}

$test1 = new Blah1();
$test1->test(); // Outputs NULL


class Blah2
{
    private $foo = NULL;

    function test()
    {
        var_dump($this->foo);
    }
}

$test2 = new Blah2();
$test2->test(); // Outputs NULL

Typed properties

PHP 7.4 adds typed properties which do not default to null by default like untyped properties, but instead default to a special "uninitialised" state which will cause an error if the property is read before it is written. See the "Type declarations" section on the PHP docs for properties.