php中数组赋值混乱求解

真抓狂了……
大意如下：

$update=['f1'=>121.231,'f2'=>2312.12,     'f3'=>31.1231,'f4'=>213.12,'fv'=>3189.21321];$month=['m1'=>123.1289,'m2'=>123.23198,      'm3'=>23.21,'m4'=>138.21,'mv'=>1231.81];      foreach($update as &$v){            $v=round($v,4);     }       foreach($month as $k=>$v){           $update[$k]=round($v,4);        }

请各位高人先看上面的代码，这个应该没有问题吧？有问题请指出来，我抓狂了，同时也不用看下面那一长大把啦的了……
核心就是将两个数组合并在一起（其他的实现方法先不谈，就只先用这个），并且保留4位小数

如果上面的代码没有问题，请看我下面的调试。
--------------------------DEBUG-------------------------
实际运行中发现fv字段还是保留了其小数位一大堆，总之没有执行round运算，让我开始怀疑人生了
代码中PR（print_r）看数据，发现在经过了update的foreach后都还是正常的，但是在进入month循环后就不正常了
整理后的调试代码见下：

if($debug_flag){            echo('原始数据');BR();          PR($update);        }       foreach($update as $k=>&$v){         $v=round($v,4);     }       //实在没有整明白为什么update的最后一个fv不进行四舍五入呢？      if($debug_flag){            echo('update foreach 循环后数据');BR();          PR($update);            echo('进入month循环');BR();         echo('month原始数据');BR();         PR($month);     }       foreach($month as $k=>$v){           if($debug_flag){                echo('in month ');BR();             PR($update);                PR($k);BR();            }           $update[$k]=round($v,4);            if($debug_flag){                PR($update);                echo('month 循环中数据');BR();           }       }       if($debug_flag){            echo('month循环后数据');BR();            PR($update);            PR($month);         die();      }

配套的调试结果见下：

原始数据Array(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 600.85900353103)update foreach 循环后数据Array(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 600.859)进入month循环month原始数据Array(    [m0] => 0    [m1] => 596.4    [m2] => 604.5    [m3] => 600.3325    [m4] => 602.0825    [mv] => 600.85900353103)in monthArray(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 0)m0Array(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 0    [m0] => 0)month 循环中数据in monthArray(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 596.4    [m0] => 0)m1Array(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 596.4    [m0] => 0    [m1] => 596.4)month 循环中数据in monthArray(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 604.5    [m0] => 0    [m1] => 596.4)m2Array(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 604.5    [m0] => 0    [m1] => 596.4    [m2] => 604.5)month 循环中数据in monthArray(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 600.3325    [m0] => 0    [m1] => 596.4    [m2] => 604.5)m3Array(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 600.3325    [m0] => 0    [m1] => 596.4    [m2] => 604.5    [m3] => 600.3325)month 循环中数据in monthArray(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 602.0825    [m0] => 0    [m1] => 596.4    [m2] => 604.5    [m3] => 600.3325)m4Array(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 602.0825    [m0] => 0    [m1] => 596.4    [m2] => 604.5    [m3] => 600.3325    [m4] => 602.0825)month 循环中数据in monthArray(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 600.85900353103    [m0] => 0    [m1] => 596.4    [m2] => 604.5    [m3] => 600.3325    [m4] => 602.0825)mvArray(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 600.85900353103    [m0] => 0    [m1] => 596.4    [m2] => 604.5    [m3] => 600.3325    [m4] => 602.0825    [mv] => 600.859)month 循环中数据month循环后数据Array(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 600.85900353103    [m0] => 0    [m1] => 596.4    [m2] => 604.5    [m3] => 600.3325    [m4] => 602.0825    [mv] => 600.859)Array(    [m0] => 0    [m1] => 596.4    [m2] => 604.5    [m3] => 600.3325    [m4] => 602.0825    [mv] => 600.85900353103)

如果您有耐心看完的话，就会发现进入month循环后数据就已经开始在乱变了……
难道foreach或者是数组的$array[$key]=$value还有什么不为人知的东西？
求援手！

回复讨论(解决方案)

调试到这里顺便就把代码改了

$update=array_merge($update,$month,$year);      foreach($update as &$v){            $v=round($v,4);     }

这个数据就是正常的了，而且也更简短+明了+没有出错！
但原来的代码是啥地方有问题啊？

$update = array (  'f1' => 596.4,  'f2' => 604.5,  'f3' => 600.3325,  'f4' => 602.0825,  'fv' => 600.85900353103,);$month = array (  'm0' => 0,  'm1' => 596.4,  'm2' => 604.5,  'm3' => 600.3325,  'm4' => 602.0825,  'mv' => 600.85900353103,);foreach($update as $k=>&$v){    $v = round($v, 4);}unset($v);foreach($month as $k=>$v){    $update[$k] = round($v, 4);}print_r($update);

Array(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 600.859    [m0] => 0    [m1] => 596.4    [m2] => 604.5    [m3] => 600.3325    [m4] => 602.0825    [mv] => 600.859)

看到区别了吗？我多用了个 unset($v);
其实这是引用惹的祸，你测试的两组数据正好是相同的，如果你把 $month['mv'] 改成 0，立刻就会发现问题了

$update = array (  'f1' => 596.4,  'f2' => 604.5,  'f3' => 600.3325,  'f4' => 602.0825,  'fv' => 600.85900353103,);$month = array (  'm0' => 0,  'm1' => 596.4,  'm2' => 604.5,  'm3' => 600.3325,  'm4' => 602.0825,  'mv' => 600.85900353103,);foreach($update as $k=>&$v){    $v = round($v, 4);}unset($v);foreach($month as $k=>$v){    $update[$k] = round($v, 4);}print_r($update);

Array(    [f1] => 596.4    [f2] => 604.5    [f3] => 600.3325    [f4] => 602.0825    [fv] => 600.859    [m0] => 0    [m1] => 596.4    [m2] => 604.5    [m3] => 600.3325    [m4] => 602.0825    [mv] => 600.859)

看到区别了吗？我多用了个 unset($v);
其实这是引用惹的祸，你测试的两组数据正好是相同的，如果你把 $month['mv'] 改成 0，立刻就会发现问题了

哦，对……忘记引用这回事情了……一般都很少用&，偶尔用用也没有出错……
谢谢！