Below taken from this tutorial
You must make sure your PHP is installed as CGI. To check do the following: Create a new file, name it info.php (just an example), and put in the following code, “”. Upload to your webserver and go to it with your browser.
Now check for Server API (4th item from the top), if it says “CGI”, you have PHP compiled as CGI, if it reads “Apache”, you have it running as an Apache module.
Now you need to check you have an extra line in your PHP script. It has to be the first line of your script and must contain your server’s PHP executable location:
That looks a lot like PERL now, doesn’t it? After that let’s add the necessary command to our crontab. Edit /etc/crontab and add the following line:
* * * * * php /path/to/your/cron.php
Execute the following from the command line:
Shell> crontab crontab
Be sure your “script.php” has the necessary permissions to be executable (“chmod 755 script.php”).
Now you are all set!
It is very possible that you're just missing the php command in your cron. You have
* * * * * root var/www/sample/index.php
but it should be
* * * * * php var/www/sample/index.php
But you must also check you have the php binary location in the PHP file as stated above.