The trade-off between php multi-level array foreach reference and copy

<code>$a = [ "a"=>["cc"=>11]  , "b"=>["cc"=>22] ];
echo "\n".$a['a']["cc"].",".$a['b']["cc"];

$i=1;

//(1)拷贝,$a无变化
foreach($a as $k=>$v) {
    $v['cc']=$i;
    $i+=1;
}
echo "\n".$a['a']["cc"].",".$a['b']["cc"];

//(2)引用
foreach($a as $k=>&$v) {
    $v['cc']=$i;
    $i+=1;
}
echo "\n".$a['a']["cc"].",".$a['b']["cc"];

//(3)拷贝
foreach($a as $k=>$v) {
    $a[$k]["cc"]=$i;
    $i+=1;
}
echo "\n".$a['a']["cc"].",".$a['b']["cc"];

</code>

<code>$a = [ "a"=>["cc"=>11]  , "b"=>["cc"=>22] ];
echo "\n".$a['a']["cc"].",".$a['b']["cc"];

$i=1;

//(1)拷贝,$a无变化
foreach($a as $k=>$v) {
    $v['cc']=$i;
    $i+=1;
}
echo "\n".$a['a']["cc"].",".$a['b']["cc"];

//(2)引用
foreach($a as $k=>&$v) {
    $v['cc']=$i;
    $i+=1;
}
echo "\n".$a['a']["cc"].",".$a['b']["cc"];

//(3)拷贝
foreach($a as $k=>$v) {
    $a[$k]["cc"]=$i;
    $i+=1;
}
echo "\n".$a['a']["cc"].",".$a['b']["cc"];

</code>