class Foo {

    private $color;

    public function bar() {
        echo 'before';
        $this->color = "blue";
        echo 'after';
    }
}

// Foo::bar();  

// Deprecated
$obj = new Foo;
$obj::bar();

Non-static method Foo::bar() should not be called statically

In PHP 7, calling non-static methods statically is deprecated, and will generate an E_DEPRECATED warning. Support for calling non-static methods statically may be removed in the future