PHP：在数组中存储多个选择值

There are maximum 8 <select name="a"> and I need to store these values in an array

$a = $_POST['a'];
$b = $_POST['b'];

if (isset($a, $b)) {
    foreach ($products as $thisProduct) {
        if ($thisProduct->getId() == $a) {
        //do something & store in array
            switch($b){
                case "one":
                //do something
                break;

                case "two":
                //do something
                break;

                case "three":
                //do something
                break;
            }
        }
    }
}
//call array and make array_sum()...

How do I store these in an array so I could use this array for making calculations?

UPDATE

There are also 8 input fields with an amount. This input multiply with the value of $a.

foreach ($products as $thisProduct) {

    foreach ($a as $value) {
        if ($thisProduct->getId() == $value) {
            $multiply = ($thisProduct->getMultiply($amount));
            array_push($array, $multiply);  
        }
    }
}

How can I link each input to the select name="[a]"?

Use <select name=a[]> and use this code

$a = $_POST['a'];
foreach($a as $value){
   // do something
}