PHP passing a value from an HTML form?

I need to add a statement to the end of this script that calls the checkgrade() function and passes to it the value from the grade form variable. Here is my code, I have updated it to show how I think it should be but I am still recieving an error.

<?php

function checkGrade($Grade) {

switch ($Grade) {

case "A":

echo "Your grade is excellent.";

break;

case "B":

echo "Your grade is good.";

break;

case "C":

echo "Your grade is fair.";

break;

case "D":

echo "You are barely passing.";

break;

case "F":

echo "You failed.";

break;

default:

echo "You did not enter a valid letter grade.";

}

$Grade = checkGrade

($Grade = $_GET["Grade"]);

echo "<p>$Grade</p>";

?>

I have an HTML form I am using that has the Get action from this php, with a single text input named Grade. Both validate on W3C. However I am getting an error parsing from the form to the function.