AFAIK cron requires that scripts run locally as root. As such anything you want cron to do, you will need to be able to do it from the commandline yourself.

So first step would be getting the process to work manually from the commandline, then adding it as a cron job will be easy (it can be done in Webmin).

With php5-cli php files can supposedly be run from the commandline (can be installed via apt-get install if not included by default - don't recall if it is). But I have only tried it once and it failed miserably (although in fairness it was most likely user error!) As the script will run from the commandline, you will need to use the filesystem location for the script (not the web location).

You'll need to do some testing and see if you can get it to work as expected and like I say once you have it working from the commandline, it should be a breeze to get it running with cron.

If you are successful be great if you can post back.

[an afterthought] Perhaps it's worth posting on the Magento forums (assuming they have forums) as perhaps there's a better/easier way to acheive what you want to do?

I'm not sure how php scripts run within Apache/php, perhaps they only require execute permissions to run via commandline? I don't know enough about the subject - guess you'll have to research and/or test. Worth a try IMO.

AFAIK root has permissions to access anything, however by default files won't have execute permission (which is what lead me to my above idea).

I spent DAYS trying to figure out cron with php, ran into SOOO much stuff with rights, and this is what i came up with and it works like a charm for me. I needed my script to run every 5 seconds, and as we know, cron is only a 1 minute minimum cycle. so...

This is how I set up a script of mine to run in cron. In the beginning of the script I set

set_time_limit(0); 

 * * * * * wget --no-check-certificate https://localhost/somepath/script.php -O /dev/null

you could use (for 3pm)

 0 15 * * * wget --no-check-certificate https://localhost/somepath/script.php -O /dev/null

Notes:

the -O is an uppercase oh

the --no-check-certificate is to bypass wget issues with SSL (not needed with http)

in this case, I run it every minute. It checks to see if it is already running, and immediately terminates if it is so I don't get more than one instance. The result is the script starting up again at the turn of the minute if it terminates for some reason.

Hope this helps. It probably took me a solid week of crud to figure this out. haha.

Yes, it is an external script (ie: accessable by a web browser from anywhere, but with a pretty garbled path)

So I could initiate it from a web browser using the whole path name of the URL

(say, www.example.com/aaa/bbb/ccc/script.php from an internet cafe in china)

or, it works with wget in a cron job on that server with localhost like...

http://localhost/aaa/bbb/ccc/script.php (as in the example I gave with all the extra stuff to pipe any output into null so it doesn't create files of any output) I did it this way so I can make a clone of the server and the script will run automatically on the clones as well without any editing.

Many would say this is a security concern, as all scripts other than public should be inaccessable from the real world, but

a) If someone did find it, there are checks that, if it's already running, it just terminates instead of making a new instance. (I do this by updating a database value with the current time and check against that)

b) If they don't have a certain value in a variable (such as ?security=7293847293) it just terminates.

So fair warning, if this is something that is a high secure script, maybe you don't want to go this "external" route.

The benefits of doing it this way is that the cron job requires no security whatsoever (wget is essentially triggering the script just like a web browser would).

I used SSH logged in as root by putty and at the command line typed:

crontab -e

This allows you to edit the crontab file directly, and does not requre a service restart or anything after you save the file. It just looks at each entry in the file at the turn of every minute (like, exactly one second after), and if any of the criteria of those first 5 fields satisfy it should run, it does.

http://www.pantz.org/software/cron/croninfo.html

This link is a good one for getting a lot of basic info about cron, in particular the last section shows exactly how to add the entries as you want them.

I have never used any other method than crontab -e at a shell prompt. It has always worked flawlessly for me and as long as it works, I just don't trust any other way.

If you need any help on editing the crontab and setting up a job, let me know, I'll help if I can.