'JsonSerializable' not found

In attempting to install Drupal 8, I get the following error:-

Fatal error: Interface 'JsonSerializable' not found in /usr/local/www/drupal8/core/lib/Drupal/Component/Render/MarkupInterface.php on line 32

Where do I begin to look? It appears to be php related, so maybe I've omitted installing one of the many php modules required to get things working...
Oops! Silly me - I forgot to create a /usr/local/etc/php.ini

How do I mark the thread solved?