Having used PHP a long time ago, this is very confusing to me. I would have thought that "${var}" does exactly the same as "$var", but being usable in more cases. For example if you want to append text without a space: "${var}moretext". This is how it works in Bash, at least.

I am really surprized that braces on the outside works at all. I would have expected "{$var}" to expand to "{FOO}".

I thought the version with parentheses is some new general-expression syntax, like "${( $foo + 2 )}" - but that doesn't seem to exist. I would have expected "${(var)}" to give a warning, like the classic "`var` is not a constant. You should use `$var`. Accessing `var` without `$` will stop working in a future version of PHP". (As I said, I used PHP a long time ago...).

PHP is indeed still "a fractal of bad design" :-D, because it gives you the illusion of being able to come back and pick it up easily. But everything does something slightly different than you expect. I commend the PHP devs for removing the footguns and making it a nicer language again!

Personally, I would have removed #2 and #4 - keeping only "$foo" and "${foo}" as equivalent, just like in Bash. And when they get a general expression syntax - maybe "$(foo)" like in Bash, or something else - then you get variable variables back for free.