多维数组上未初始化的字符串偏移量

Question

Any idea why lines #2 and #6 throw a Notice: Uninitialized string offset: 0?

function get_user_class() {
    if ((int) $GLOBALS['CURUSER']['class'] > $GLOBALS['UC_HIGHEST_CLASS'] || (int) $GLOBALS['CURUSER']['class'] < 0) {
        exit();
    }
    else
        return $GLOBALS['CURUSER']['class'];
}

---

var_dump($GLOBALS['CURUSER']['class'])

gives:

string '0' (length=1)

---

php -v

is:

PHP 5.2.6-1+lenny13 with Suhosin-Patch 0.9.6.2