You should destroy the $img too...
So it would be :
$img = @imagecreatefromjpeg("http://www.mysite.com/my_image.jpg");
if ($img) {
$img_height = imagesy($img);
ImageDestroy($img);
}
echo "My height is " . $img_height;
boo
To use this function notice that 'image' parameter it's a RESOURCE and NOT an Image File Path !
Here comes an exemple:
$img = @imagecreatefromgif("http://www.mysite.com/my_imag.gif");
if ($img) $img_height = imagesy($img);
echo "My height is " . $img_height;