file_exists使用字符串，但不是$ var

So I'm trying to make a dynamic link page that takes a load of entries, links to an image if it exists, and links to an upload box if it doesn't. For some reason, though, when I pass a variable to file_exists it returns false, even if passing the same location directly returns true. For example:

$tempcomp = "dogs/big dogs/misty.puppy.jpg";

                if (file_exists($tempcomp))
                    {
                        echo "YES1";
                    }
                clearstatcache();
                if (file_exists ("dogs/big dogs/misty.puppy.jpg"))
                    {
                        echo "YES2";
                    }
                clearstatcache();

This code returns YES2, even though both statements are identical. What causes this kind of behavior?