Running a PHP script in unix background

Hi,
I am new to unix, I am going to run a PHP script in the background by using the following shell command,

exec("/usr/bin/php path/to/script.php >/dev/null &");

PHP:

What my doubt is:

1]How many times the script.php will be executed? [will it execute repeatedly or once]
2]Should I stop this script after completion of my job?

Thanks in advance
tamilsoft

 

1. Once
2. yes