The difference between NULL and no value assignment in the PHPClass attribute

When PHP defines the Class attribute, what is the difference between NULL and no value assignment? For example: Code 1: {code...} Code 2: {code...} What is the difference between the above Code? When PHP defines the Class attribute, what is the difference between NULL and no value assignment?
For example:
Code 1:

class Base{    protected $_db = NULL;       function __construct()    {        }}

Code 2:

class Base{    protected $_db;       function __construct()    {        }}

What is the difference between the above Code?

Reply content:

When PHP defines the Class attribute, what is the difference between NULL and no value assignment?
For example:
Code 1:

class Base{    protected $_db = NULL;       function __construct()    {        }}

Code 2:

class Base{    protected $_db;       function __construct()    {        }}

What is the difference between the above Code?

It is a good habit to declare the initial value of a variable.

No difference

Http://php.net/manual/zh/language.types.null.php

Class Base {protected $ _ db = NULL; function _ construct () {}}$ B = new Base (); var_dump ($ B); output: object (Base) #1 (1) {["_ db": protected] => NULL} class Base {protected $ _ db; function _ construct () {}}$ B = new Base (); var_dump ($ B); also output: object (Base) #1 (1) {["_ db ": protected] => NULL} but when you do not define a variable $ var; $ var_dump ($ var) directly in the class, a warning of Notice: Undefined variable: is returned, but the output is null.