如何创建动态数组变量

I have these arrays and i want to loop through every array by creating a dynamic variable that put their name in foreach loop automatically something like this

foreach ($activities.$i as &$activity) { //$i = 1,2,3,4..
   //code

}

//activities
$activities1 = $_POST["activities1"];
$activities2 = $_POST["activities2"];
$activities3 = $_POST["activities3"];
$activities4 = $_POST["activities4"];

Easier method is to simply use the array naming hack:

<input name="activities[1]" ..>
<input name="activities[2]" ..>
<input name="activities[3]" ..>

which makes $_POST['activities'] you array:

foreach($_POST['activities'] as $i => $value) {
   // $i -> 1,2,3,4
}

But if you insist on embedding the index inside the key's name, then:

foreach(range(1,4) as $i) {
    foreach($_POST["activities{$i}"] as $value) {
        ...
    }
}