As far as I know, 7.0.18 is newer than 5.5. 
How do I resolve this?
As discussed many times on this community forum.
PHP 7 is not supported for any current Xibo release. 1.8 series actively warns about this, and we are updating the documentation to more accurately reflect that - http://xibo.org.uk/manual/en/manual_install.html ie (lower than PHP7)
For now, you should downgrade the php version to 5.6.