at your command line:
crontab -e (edit your crontab, opens VIM editor)
Then hit 'i' to go into insert mode (should show --INSERT -- at the bottom) and type this into the line:
*/5 * * * * php /home/xyz/apps/app/scheduled.php > /dev/null 2>&1
then hit ESC, to exit insert mode.
then type :w (to write the file) and ENTER
then type :q (to quit the editor) and ENTER
To verify, from command line run: crontab -l (lists the contents of the file without editing).
edit: ninja'd by peter