Osmc is built on standard debian jessie. So yes, not everything included in raspbian might be available.
So some tutorials based on raspbian might not work.
Edit: This tutorial suggests to add the raspbian unstable branch to the sources. This is unsupported and might potentially break your system.
It might work though. Be sure to remove the foreign repository again after installation.
But only on your own risk.
I’d use the php version available from the psmc/debian jessie repository.
Also raspiconfig will not work and firmwareupdates are managed by the osmc update system.