C++-like function-try-block proposal

It would blur the concept of a function body scope, and thus the mental model of how the language works.
E.g. are local variables and parameters shared between the try {} and the catch {} block?

PHP documentation stresses it that the scope is always the entire function, i.e. it isn't like C/C++ where each compound statement makes up a new scope. The construct may indeed look alien to those who haven't had experience with C++, however, I believe this is a matter of a single clarification statement about scope, plus a tip like "just think of it as if wrapping curly braces are there around it, implicitly" (which is in fact the case), and soon you'll find yourself comfortable with it.

It would also mean that userland PHP parsers and IDEs need to support the syntax.

True. However, this is true for pretty much any language change, and PHP has seen quite a few lately (typehints, anonymous classes.) I don't think we should be worried about changes other projects would have to make to cope up with language evolution lest it becomes an (unnecessary and surrogate) obstacle. My vision is that if some company/group produces an IDE, it is but natural to expect them to be ready to adapt to language changes.