我想在php中制作搜索过滤器。 会话第一次不起作用，但在第二个值传递时它正常运行

I am making search filter in php. But session is not working for first time. Its running properly after second time submission. the coding is like this:

session_start();
include('connect.php');
$select = "SELECT *";
$from = " FROM filter";
if($_POST)
    {
        $_SESSION['days'] = array();
        $_SESSION['WHERE'] = array();
        $day = $_POST['day'];
        $WHERE = $_SESSION['WHERE'];
        if($day!='')
            {
                foreach($day as $k)
                    {
                        array_push($_SESSION['days'],$k);
                        $Day[] = "dayId = $k";
                    }
                $WHERE[] = ' ('.implode(' OR ',$Day).')';
            }
        else
            {
                $WHERE[] = '';
            }
    }
if(!empty($_SESSION['days']))
    {
        foreach($_SESSION['days'] as $l)
            {
                ${m.$l} = 'checked = "checked"';
            }
    }