class PluginA {
  public function func_a() {
    // do stuff
  }
}

class PluginB {
  function functB() {
    if (class_exists('PluginA')) {
      //do stuff that depends of PluginA
    }
  }
}

add_action('plugins_loaded', 'call_plugin_a_using_plugin_b');
function call_plugin_a_using_plugin_b() {
  PluginB::functB();
}

function from_plugin_alpha( some_thing ) {
    // do stuff
}

function from_plugin_bravo() {
    if ( !function_exists( 'from_plugin_alpha' ) ) {
        return;
    }
    from_plugin_alpha( some_thing );
}