gistfile1.php

<?php

class super_kewl {
  // can be public or no visibility declaration (which defaults to public)
  public function matt($param = false) {
    echo $param ? esc_attr($param) : 'Hello!';
  }
}

// will output Hello
super_kewl::matt();

// or, if you want it to look different
function matt_is_kewl($param) {
  super_kewl::matt($param);
}

matt_is_kewl();


// however, if you have $this, you can't do that
class bad_code {
  private $property = 'Hello';
  public function bad_method($param = false) {
    echo $this->property . ($param ? esc_attr($param) : ' World!');
  }
}

// will fail
bad_code::bad_method();