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


///////////////////////////////////////////////////////
script executer toutes les minutes

crontab -e

#min hour day month weekday command
*/1   *    *    *    *     <your command>

toutes les 5 monutes

5 * * * * /home/aspera/my_script.sh



/15 * * * * fswebcam -r 960x720 -d /dev/video0 /home/pi/webcam.jpg


http://techjp.net/raspberry-pi-serveur-de-surveillance/

http://blog.davidvassallo.me/2014/03/15/raspberry-pi-home-control-center-part-2/

http://nojhan.free.fr/article.php3?id_article=22