<?php    
    $x = false;
    $y = 0;
    if (isset ($_POST ["myform"]))
    {
        $x = $_POST['']?? false;
        is_array($x);
        $summa = 0;
        if (is_array($x))
        {
        foreach ($_POST[''] as $y)
            $y += $y;
        }
    }
?>
<?php if ($y !== false) : ?><p>Сумма равна: <?=$y?></p><?php endif ?>

form name="myform" action="<?=$_SERVER['REQUEST_URI']?>" method="post">
    <div>
        X: <input type="textarea" name="x" value="<?=$x?>" />
    </div>
    <div>
        <input type="submit" name="myform" value="Сумма" />
    </div>
</form>