变量在If语句中不起作用

As I posted here, I managed to get 2 variable that I would like to use for further processing. Unfortunatly, I can't use is inside that if statement.

if (isset($_POST['modifybtn'])) { //Now we have our variables all set
    $SubmissionID = $_POST['modifybtn'];
    $CourseID = $_POST['courseid'];
    }
//We do some irrelevant stuff here.... and then

echo $CourseID; //Works
if (isset($_POST['savebtn'])) {
        echo $CourseID; //Not Working

    saveData($AppID, $CourseID);
    header("Location: applicantCase.php");
}
if (isset($_POST['nextbtn'])) {
    saveData($AppID, $CourseID);
    header("Location: ApplicantApplyEducation.php");
}

Why is that happening, one line before was working?

First, You can define outside the if condition.

 $SubmissionID = "";
 $CourseID = "";
if (isset($_POST['modifybtn'])) { //Now we have our variables all set
    $SubmissionID = $_POST['modifybtn'];
    $CourseID = $_POST['courseid'];
    }
//We do some irrelevant stuff here.... and then

echo $CourseID; //Works
if (isset($_POST['savebtn'])) {
        echo $CourseID; //Now it will Working

    saveData($AppID, $CourseID);
    header("Location: applicantCase.php");
}
if (isset($_POST['nextbtn'])) {
    saveData($AppID, $CourseID); // Now it will work
    header("Location: ApplicantApplyEducation.php");
}