How to use $_GET variable

<?php
    if(isset($_GET['day']) && isset($_GET['date']) && isset($_GET['year'])){
       
        $day=$_GET['day'];
        $date=$_GET['date'];
        $year=$_GET['year'];
   
        if(!empty($day) && !empty ($date) && !empty ($year)){
            echo "ok";
        }
       
        else{
            echo "please fill the field";
        }
    }


?>

<form action ="get_variable.php" method="GET">
    Day: <br /><input type="text" name="day"><br />
    Date: <br /><input type="text" name="date"><br />
    Year: <br /><input type="text" name="year"><br /><br />
    <input type="submit" value="submit">
</form>