Everyone knows cronjob!
You have simply to do
man 5 crontab
the former will give a general description of the crontab, the latter will give a description of the crontab file format.
Regarding your requirements, you can edit your crontab (crontab -e) and add an entry like this:
*/30 * * * * /path/to/php spider.php
where the first field (minutes) means "every 30 minutes". Substitute the /path/to/php with the real path on your machine and the trick is done.