PHP如何保存用户在变量中输入的日，月和年，以在数据库中显示值

>用户输入日期，月份和年份，然后按一个按钮根据他选择的日期向他显示数据库的值，但我不知道如何在$d，$m和$y变量中保存日期，月份和年份

<style>
.button {border-radius: 8px;} 
.button1 {font-size: 20px;}
</style>
<form>
اليوم : <input id="txtDay" type="text" placeholder="DD" />
الشهر : <input id="txtMonth" type="text" placeholder="MM" />
السنة : <input id="txtYear" type="text" placeholder="YYYY" />
<button id="but" onclick="daily()">إظهار الإستهلاك اليومي</button>
</form>
<script>// <![CDATA[
// to show the right value from database
  function daily() {
  $current_user=get_current_user_id();// to fetch the right data for the right user
  // takes the values ​​entered by the user
global $wpdb;
$d=???????????//define variable to store the day enter by user 
$m=???????????//define variable to store the month enter by user 
$y=???????????//define variable to store the year enter by user 
$daily_amount= $wpdb->get_var("SELECT daily_amount FROM arduino_period_use where ID=$current_user and day=$d and month=$m and year=$y ");
print_r($daily_amount); 
}
// ]]></script