PHP Weirdness: Calling Non-Static Methods Statically

exhibit1.php

<?php
class A {
	public function foo() {
		echo "Hello World";
	}
}
A::foo(); // => Hello World

exhibit2.php

<?php
class A {
	public $bar = 42;
	public function foo() {
		echo $this->bar;
	}
}
A::foo(); // => Fatal error: Using $this when not in object context in /code/Yk6d7i on line 5

exhibit3.php

<?php
class A {
	public $bar = 42;
	public function foo() {
		echo $this->bar;
	}
}
class B {
	public $bar = 23;
	public function __construct() {
		A::foo();
	}
}
new B; // => 23