How did i get and Session Var into an function

my problem is that i did not get an php session variable into my function – i start in the top of the file

<?php
session_start();
var_dump($_SESSION);

this is the result

array(3) { ["loggedin"]=> bool(true) ["name"]=> string(4) "Piet" ["id_staff"]=> int(9) }

public static function getStaffList($_SESSION['name']){

  if ($_SESSION['name'] =="admin" || $_SESSION['name'] =="Piet") {
    $sql = "SELECT * FROM staff";
  }
  else {
    $sql = "SELECT * FROM staff where surname = '".$_SESSION['name']."'";
  }

  $result = Core::$link->query($sql);

  if (!$result) {
    return 'Error: ' . mysqli_error(Core::$link);
  }

  $return = array();
  while ($myrow = mysqli_fetch_assoc($result)) {
    if ($myrow['birthday'] !== '0000-00-00') {
      $myrow['birthday'] = date("d.m.Y", strtotime($myrow['birthday']));
    } else {
      $myrow['birthday'] = '';
    }

    $return[] = $myrow;
  }

  return $return;
}