PHP: check if point/coordinate inside of ellipse

Question

How can I check if a Point is inside a Ellipse? When i draw a Ellipse like the this:

imageellipse($image, 300, 460, 400, 590, $col_ellipse);

How can I find out if the Point x = 100 y = 400 is inside my Ellipse?

RESULT:

function posInside($x, $y, $h, $k, $rx, $ry) {
    /*$h = 400; //center x of ellipse
    $k = 960; //center y of ellipse
    $rx = 400 / 2; //radius x
    $ry = 590 / 2; //radius y */
    $part1 = pow(($x - $h) / $rx, 2);
  $part2 = pow(($y - $k) / $ry, 2);
    if($part1 + $part2 <= 1) {
        return true;
   } else return false;
}

Answer 1

Thank you to @Jordi Nebot for the help with the math form. My Result:

function posInside($x, $y, $h, $k, $rx, $ry) {
    /*$h = 400; //center x of ellipse
    $k = 960; //center y of ellipse
    $rx = 400 / 2; //radius x
    $ry = 590 / 2; //radius y */
    $part1 = pow(($x - $h) / $rx, 2);
  $part2 = pow(($y - $k) / $ry, 2);
    if($part1 + $part2 <= 1) {
        return true;
   } else return false;
}