Does/Why php forces you to use the object constructor - php

Question

I have been doing some research on objects in PHP. All the examples I have seen use the object constructor even on their own objects. Does PHP force you to do this and if so why?
For example:
<?php
class Person {
public $isAlive = true;
public $firstname;
public $lastname;
public $age;
public function __construct($firstname, $lastname, $age) {
$this->firstname = $firstname;
$this->lastname = $lastname;
$this->age = $age;
}
public function greet() {
return "Hello, my name is " . $this->firstname . " " . $this->lastname . ". Nice to meet you! :-)";
}
}
// Creating a new person called "boring 12345", who is 12345 years old ;-)
$me = new Person('boring', '12345', 12345);
echo $me->greet();
?>
But if I do this:
<?php
class Person {
public $isAlive = true;
public $firstname;
public $lastname;
public $age;
}
$person->firstname = "John";
echo $person->firstname;
?>
I get a http error code 500.(ie: My code crashed).

Answer 1

You're incorrectly associating the __construct() function with the way in which you instantiate a class/object.
You don't have to use the __construct() function (it's optional). However, before you can use a class's methods, you do have to create an instance of it first.
<?php
class Person {
public $isAlive = true;
public $firstname;
public $lastname;
public $age;
}
$person = new Person(); //Add this line
$person->firstname = "John";
echo $person->firstname;
?>

Answer 2

Because a class definition is exactly that, a definition.
To create an instance of a class (an actual object), you have to instantiate it. You may in fact instantiate many different objects/instances of the same class.
And you don't have to define a constructor method..... but you do have to instantiate using new

Answer 3

There are two ways to work with the class variables.
1. the variable should be public static like:
class Person {
public $isAlive = true;
public static $firstname;
public $lastname;
public $age;
}
Person::$firstname = "John";
echo Person::$firstname;
2. the access to variable via the class object
class Person {
public $isAlive = true;
public $firstname;
public $lastname;
public $age;
}
$person->firstname = "John";
echo $person->firstname;
By the way, PHP can create the object by itself php 5.5 at least.
Your code is not crashing. It just gives a Warning:
Warning: Creating default object from empty value in ...
So WHY?
This is how PHP working with the memory. In a first case the static means that php already allocated memory for the variable and you can work with it. The second case. By using NEW command you give instruction to php to allocate memory and load class into it. So, after the class was created you have an access to the var.
In a third case PHP is calling new Person by itself and give you the warning. I am strongly not recommend to count on php default behavior. ALWAYS create an object in explicit way