php 奇怪现象，求大神解答

$aBU = array (
array ( 'id' => '0', ),
array ( 'id' => '1', ),
array ( 'id' => '2', ),
array ( 'id' => '3', ),
);
echo "
原来数据";
print_r($aBU);
echo "
";
foreach($aBU as  &$bu){
$bu['dept'] = rand(100, 999);
}
//unset($bu);
echo "
加入dept后";
print_r($aBU);
echo "
";
echo "
重新遍历";
foreach($aBU as $key => $bu){
    print_r($bu);
echo "
";
}

输出：

原来数据

Array( [0] => Array ( [id] => 0 ) [1] => Array ( [id] => 1 ) [2] => Array ( [id] => 2 ) [3] => Array ( [id] => 3 ))

---

加入dept后

Array( [0] => Array ( [id] => 0 [dept] => 783 ) [1] => Array ( [id] => 1 [dept] => 683 ) [2] => Array ( [id] => 2 [dept] => 442 ) [3] => Array ( [id] => 3 [dept] => 121 ))

---

重新遍历，最后一个元素的竟然得到的竟然是前一个元素？

Array( [id] => 0 [dept] => 783)
Array( [id] => 1 [dept] => 683)
Array( [id] => 2 [dept] => 442)
Array( [id] => 2 [dept] => 442)

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