php strange phenomenon, please give me an answer

<span><?php
</span><span >$aBU </span><span >= </span><span >array </span><span >(
</span><span ></span><span >array </span><span >( </span><span >'id' </span><span >=> </span><span>'0'</span><span>, </span><span>)</span><span>,
</span><span></span><span>array </span><span>( </span><span>'id' </span><span>=> </span><span>'1'</span><span>, </span><span>)</span><span>,
</span><span></span><span>array </span><span>( </span><span>'id' </span><span>=> </span><span>'2'</span><span>, </span><span>)</span><span>,
</span><span></span><span>array </span><span>( </span><span>'id' </span><span>=> </span><span>'3'</span><span>, </span><span>)</span><span>,
</span><span>)</span><span>;
</span><span>echo </span><span>"<h3></span><span color: background-color:>原来数据</span><span></h3>"</span><span>;
</span><span>print_r(</span><span>$aBU</span><span>)</span><span>;
</span><span>echo </span><span>"<hr />"</span><span>;
</span><span></span><span>foreach</span><span>(</span><span>$aBU </span><span>as  </span><span>&</span><span>$bu</span><span>){
</span><span></span><span>$bu</span><span>[</span><span>'dept'</span><span>] = rand(</span><span>100</span><span>, </span><span>999</span><span>)</span><span>;
</span><span>}
</span><span>//unset($bu);
</span><span>echo </span><span>"<h3></span><span color: background-color:>加入</span><span>dept</span><span color: background-color:>后</span><span></h3>"</span><span>;
</span><span>print_r(</span><span>$aBU</span><span>)</span><span>;
</span><span>echo </span><span>"<hr />"</span><span>;
</span><span></span><span></span><span>echo </span><span>"<h3></span><span color: background-color:>重新遍历</span><span></h3>"</span><span>;
</span><span>foreach</span><span>(</span><span>$aBU </span><span>as </span><span>$key </span><span>=> </span><span>$bu</span><span>){
</span><span>    print_r(</span><span>$bu</span><span>)</span><span>;
</span><span></span><span>echo </span><span>"<br />"</span><span>;
</span><span>}</span>

Output:

Original data

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

---

After adding dept

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 ) )

---

Traverse again, The last element actually got the previous element?

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

The above has introduced the strange phenomenon of PHP. Please ask the master to answer it, including the relevant content. I hope it will be helpful to friends who are interested in PHP tutorials.