This is the format of /etc/crontab:
# .---------------- minute (0 - 59)
# | .------------- hour (0 - 23)
# | | .---------- day of month (1 - 31)
# | | | .------- month (1 - 12) OR jan,feb,mar,apr ...
# | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * * user-name command to be executed
I recommend copy & pasting that into the top of your crontab file so that you always have the reference handy. RedHat systems are setup that way by default.
To run something every minute:
* * * * * username /var/www/html/a.php
To run something at midnight of every day:
0 0 * * * username /var/www/html/reset.php
You can either include /usr/bin/php in the command to run, or you can make the php scripts directly executable:
chmod +x file.php
Start your php file with a shebang so that your shell knows which interpreter to use:
#!/usr/bin/php
<?php
// your code here