problem with changing image name

[tom liu]

I want to write a php file to let user upload image. But i am afraid that they will upload the image with same name. So the new one will overwrite the old one. I want to add time for each image name(i.e. test2000-1-1.gif). What shall i do? Many thanks in advance.
Below is source code.

if ($_FILES['imagefile']['type'] == "image/pjpeg"||"image/gif"){
copy($_FILES['imagefile']['tmp_name'], "C:/apache/htdocs/upload/image/".$_FILES['imagefile']['name'])
or die ("Could not copy");
echo "Thanks";

}
else {
echo "<br><br>";
echo "Could Not Copy, Wrong Filetype (".$_FILES['imagefile']['name'].")<br>";
}