Center watermark on image

Center watermark on image

I need to add watermark on image. I have solved using this code, work
well, but image is positioned on left/bottom corner. How to set to center
watermark on image center?
$img = 'test.jpg';
// Load the image where the logo will be embeded into
$image = imagecreatefromjpeg($img);
// Load the logo image
$logoImage = imagecreatefrompng("watermark.png");
imagealphablending($logoImage, true);
// Get dimensions
$imageWidth=imagesx($image);
$imageHeight=imagesy($image);
$logoWidth=imagesx($logoImage);
$logoHeight=imagesy($logoImage);
// Paste the logo
imagecopy(
// destination
$image,
// source
$logoImage,
// destination x and y
$imageWidth-$logoWidth, $imageHeight-$logoHeight,
// source x and y
0, 0,
// width and height of the area of the source to copy
$logoWidth, $logoHeight);
// Set type of image and send the output
header("Content-type: image/png");
imagePng($image);
// Release memory
imageDestroy($image);
imageDestroy($imageLogo);