0 12 * * * root php /srv/www/mysar/dump.php >> /dev/null 2>&1
|3||Day of month (1-31)|
|4||Month (1-12, Jan, Feb, etc)|
|5||Day of week (0-6) 0 = Sunday, 1 = Monday etc or Sun, Mon, etc)|
|6||Run the command as following user|
|7||Command to execute|
Now to control the access to crontab for particular user you need to edit the required configuration file depending upon your requirement, which are mentioned below.
If it exists, a user must be listed in this file in order to use crontab.
If it exists, a user must not be listed in this file in order to user crontab.
Similary for "at" access control, you can edit the following files depending upon your requirement
Same as cron.allow, only for "at".
Same as cron.deny, only for "at".