哪个更好foo :: bar（）VS $ foo :: bar（）？

About this:

class foo {
    public static function bar() {
        echo 'hi';
    }
}

What's difference between this:

foo::bar();

and this:

$obj = new foo();
$obj::bar();

Or not difference? Are both the right and the principles? Which is better?

I believe that there is no difference between them, but from my experience most often used form is Foo::bar().

You can find some examples here.

There is static method example with usage:

<?php

  class Foo {
     public static function aStaticMethod() {
      // ...
     }
  }

  Foo::aStaticMethod();
  $classname = 'Foo';
  $classname::aStaticMethod(); // As of PHP 5.3.0

?>

After that you can find example with accessing properties:

print Foo::$my_static . "
";

$foo = new Foo();
print $foo::$my_static . "
";

It means that both ways are correct. It's up to you what to use.