Before install, you need that the ORACLE_CLIENT_HOME
environment variable has been loaded correctly, that
variable is set in profile.d file, from
oracle-instantclient-basic package, that is a requirement
of oracle-instantclient-devel package.

You can logout and login again or simply run:

# source /etc/profile.d/oracle-instantclient-basic.sh

To enable the oci8 extension for php, go to
/etc/php/oci8.ini and uncomment the line:
  ; extension=oci8.so