SOLVED
Just been Googling (to preempt possible hosting company objections to enabling the function)… Looks like it was possibly disabled due to a vulnerability in php 5.3. I’m guessing this vulnerability is now patched in current php version, but php.ini has not been updated accordingly.
Thank you sendmecakes. My problem solved .
I changed the php version of my host from CPANEL settings from 5.3 to 5.6 , and the error is gone. (by changing to 5.4 or 5.5 the error exists)