double amount by day

<form action="" method="post" enctype="multipart/form-data">
    <label for="amount">Enter Amount</label>
    <input type="text" name="amount">
    <input type="hidden" name="today" value="<?php echo strtotime("now"); ?>">
    <input type="submit" name="submit" value="Submit">
</form>

<?php
if(isset($_POST['submit']) && $_POST['submit'] == 'Submit') :
    $strtoday = $_POST['today'];
    $deadline = strtotime("31 March 2020");
    $today = date("Y-m-d", $strtoday);
    $endday = date("Y-m-d", $deadline);
    $secs = $deadline - $strtoday;
    $totalday = floor($secs / (24 * 60 * 60 ));
    $amount = $_POST['amount'];
    $array = array($today => $amount);
    echo "Today: ".$today."<br><br>";
    echo "Deadline: ".$endday."<br><br>";
    echo "Total Days: ".$totalday."<br><br>";
    //echo $today. " &amp; Amount = ".$amount."<br>";
    for($i=0; $i<=$totalday; $i++) {
        $next = strtotime("+1 day", $strtoday);
        $nextday = date("Y-m-d", $next);
        $nextamount = $amount * 2;
        $strtoday = $next;
        $amount = $nextamount;
        $array[$nextday] = $nextamount;
        //echo $nextday. " &amp; Amount = ".$nextamount."<br>";
    }
    foreach( $array as $key => $value ){
        echo $key."\t=>\t".$value."\n";
    }
endif;
?>