As others have said, cron
is not something you'd call within PHP, which explains why you haven't found any tutorials for doing it.
The question is: what exactly are you trying to do? Do you want to run a PHP program on a regular interval? If that's what you want to do, then yes, you'll need cron, but nothing about it would be PHP specific, so just look for cron tutorials on how to format the crontab files.
If you want to configure your cron setup within PHP (which is how it reads from your question), then you'll need to modify the crontab
file. This is a simple text file, so loading it and saving it would be as easy as any other text file. (You'll likely have issues with file permissions, but that's something you should probably get help with separately).
To modify the crontab settings, you'll need to know know how to format the entries in the crontab file, so again the same cron-specific tutorials I mentioned above will be useful (ie don't need to include 'PHP' in your search). Then write a php program that can save data in that format. It's actually not a very complicated format, so if you know PHP reasonably well, you should be able to work out how to parse the data. (but let us know if you need further assistance with that)
Hope that helps you get started.