The latest formatting recommendation from the PSR-12 is to always use 4 spaces to indent. While I'm a fan of tabs myself, we should consider whether we want to change the recommendation on the PHP subpage to match the PSR-12.
Topic on Manual talk:Coding conventions
No. There is no meaningful interoperable benefit from which way to use spacing. Since the early 21st century, humanity has decided to indefinitely leave this question unanswered by forever more supporting both in the abstract. This is solved through EditorConfig and PHPCS. I don't expect contributors or text editor software to benefit from this switch, and more generally we don't use the rest of PSR coding style either. I'm boldly closing this as this would very likely otherwise become a distraction and waste of time, so I'm raising the bar a little by requiring the next person to re-open this if they feel compelled to convince others to spend time on this.