First of all, Nextcloud 15 doesn’t support PHP 5.x anymore. You need to update PHP to at least v7.0 as you can see here.
A description how to setup Nextcloud on CentOS 7.5 woth PHP 7.2 can be found here.
If you select the search function in the upper right corner, you will find several similar questions asked and already answered in this forum, like e.g.: