I seem to recall seeing something like this in one of my many attempts at installing Flarum. Make sure you have all the required PHP extensions installed and enabled.
I see a couple of items in that list that I have encountered. Such as, xml, dom, json and tokenizer. See if these are installed and enabled. I think you can run php -i. You should see something like json support => enabled for example, for enabled extensions.
It will be a long list so you might want to cut out the bulk with grep such as php -i | grep "enabled". Just be aware of capitalization, i.e. it might say "Enabled" instead of "enabled" and you will miss that, so adjust accordingly and repeat the command or find a better solution.
