Tomas Vortuba has put together a post sharing a summary of what's new in PHP 7.3 using a bit different tactic than just descriptions: via diffs (in about 30 seconds).

No time but eager to hear PHP news? PHP 7.3 is out in December 2018 and it brings 173 changes. Which are the most useful ones?

For each item in his list he provides code snippets showing the change for:

• Comma After the Last Argument
• First and Last Array Key
• Countable for Risky Variables
• Safer JSON Parsing

Each item on the list also links over to the related RFC for the feature that provides more detail on the change.