Basically you will want to run a PHP script file in specific intervals. Suppose you want to execute a php file called maintanence.php every one hour. This is what you do :-
The CRON Command is in the Following Format
CODE
[ Minute - Hour - Day - Month - Weekday ] - Command
The COMMAND, can be broken down in
CODE
[PATH OF PHP] [ARGUMENTS] [PATH OF PHP SCRIPT]
So the COMPLETE CRON command can be written as
CODE
[ Minute - Hour - Day - Month - Weekday ] [PATH OF PHP] [ARGUMENTS] [PATH OF PHP SCRIPT]
The timing is spedified using * symbols
CODE
* * * * * => Execute every minute
0 * * * * => Execute every Hour
0 0 * * * => Execute every mid-night
0 0 0 * * => Execute every Month
0 0 0 0 * => Execute every Weekday
If you did not understand anything till now.. Good, this means you are a noob.. Read ahead 
Since this is a UNIX command, You will have to Mention the PATH of PHP. At Astahost,
CODE
PATH TO PHP : /usr/local/bin/php
( it also same at trap17, if users over there want to use it )
These are the Possible Command line Arguments you can use. This will effect the output. In our case, we will use the -q ( Quiet mode ) argument.
CODE
-a Run interactively
-b <address:port>|<port> Bind Path for external FASTCGI Server mode
-C Do not chdir to the script's directory
-c <path>|<file> Look for php.ini file in this directory
-n No php.ini file will be used
-d foo[=bar] Define INI entry foo with value 'bar'
-e Generate extended information for debugger/profiler
-f <file> Parse <file>. Implies `-q'
-h This help
-i PHP information
-l Syntax check only (lint)
-m Show compiled in modules
-q Quiet-mode. Suppress HTTP Header output.
-s Display colour syntax highlighted source.
-v Version number
-w Display source with stripped comments and whitespace.
-z <file> Load Zend extension <file>.
The Path of the PHP file Must be Complete absolute path.
If you have an Account at TRAP, And your USERNAME is "tom", Your path will be
"/home/tom/public_html/"
I assume you are familier with the PUBLIC_HTML directory. That is the Root folder where you store your html files.
So lets say that Tom wants to execute my script "maintenance.php" every hour. And it is stored in "public_html/cron/maintenance.php";
So the Complete CRON command would be,
CODE
0 * * * * /usr/local/bin/php -q /home/tom/public_html/cron/maintenance.php
If tom wants to execute it every minute, he would use.
CODE
* * * * * /usr/local/bin/php -q /home/tom/public_html/cron/maintenance.php
If tom wants to execute it every Month, he would use.
CODE
0 0 0 * * /usr/local/bin/php -q /home/tom/public_html/cron/maintenance.php
There are more Complex forms of Assigning the TIMINGS for these scripts. You can go to CPANEL => Cron Jobs => Standard and set exact time when the script will be executed.
Reference: http://www.trap17.com/index.php/cron-jobs-cron-jobs-cpanel_t6321.html
