PHP Classes: when to use :: vs. ->?

Question

I understand that there are two ways to access a PHP class - \"::\" and \"->\". Sometime one seems to work for me, while the other doesn\'t, and I don\'t understand why.

Answer 1

It should also be noted that every static function can also be called using an instance of the class but not the other way around.

So this works:

class Foo
{
  public static function bar(){}
}

$f = new Foo();
$f->bar(); //works
Foo::bar(); //works

And this doesn't:

class Foo
{
  protected $test="fddf";
  public function bar(){ echo $this->test; }
}

$f = new Foo();
$f->bar(); //works
Foo::bar(); //fails because $this->test can't be accessed from a static call

Of course you should restrict yourself to calling static methods in a static way, because instantiating an instance not only costs memory but also doesn't make much sense.

This explanation was mainly to illustrate why it worked for you some of the times.