a::extract PHP Méthode

    static function extract($array, $key)
    {
        $output = array();
        foreach ($array as $a) {
            if (isset($a[$key])) {
                $output[] = $a[$key];
            }
        }
        return $output;
    }

 public function testExtract()
 {
     $users = $this->users;
     $usernames = a::extract($users, 'username');
     $this->assertEquals(array('peter', 'paul', 'mary'), $usernames);
 }