If语句在php/html/javascript上不起作用.总是错误的

我对下面的代码有问题。为什么当我测试$date变量时，if语句的返回值总是为真?这段代码的目的是在第一次加载页面时检查$date变量是否为空。如果页面是第一次加载，$date变量将返回当前日期，如果我从form中选择了一个日期，它将加载相同的页面，变量$date将是选择的日期。

        <form action="#" method="POST">
            <p>Select date: <input type="text" id="datepicker" name="datepicker" style="display: inline !important; width: 100px;">
                <input type="submit" value="Ok"></p>
                    <?php
                    //Variable to return the value of datepicker (calendar)
                        if (empty($date)) { //condition to check if the $date variable is empty, if empty select current date and send value to another php script
                            $date=date("Y-m-d")."%";    //select current date
                            $_SESSION['sending_date'] = $date;
                        } else{// if the value of date was selected, send variable value to another php script
                                $_SESSION['sending_date'] = $_POST['datepicker'] . "%";
                            }   
                    ?>
                <div id="draw_chart"></div> 
        </form>