If you just want the directory (without the file name) you could add:
echo dirname(__FILE__);
If you want to know in what directory the script is currently running (in certain situations it may not be the same as where the script file lives):
echo getcwd();
If you want to know the real path of any of these (e.g. after resolving all symbolic links), you could wrap them in [man]realpath/man:
echo realpath(__FILE__);