Re: PHP mode and mmm-mode

[martin rudalics]

The most simple approach would be to mark regions reserved for "other"
modes by turning them into generic strings.  Usually, major modes don't
touch strings during indentation, font-lock colors them uniformly,
parse-partial-sexp and syntax-ppss handle them.  Hence there wouldn't
have to be done too much to implement this - point-entered/-left hooks
to switch to the appropriate mode and set the appropriate syntax-table
properties.  Obviously, a region reserved for a particular mode had to
maintain point-entered/-left properties appropriately - likely the more
expensive overhead involved.

With foo-mode active, bar-mode and baz-mode would be barred off as

foo-mode
|bar-mode|
foo-mode
|baz-mode
bar-mode|

where text between two matching bars is a generic string.  Entering
bar-mode text would require to reinstall syntax-table text properties

|foo-mode|
bar-mode
|foo-mode
baz-mode|
bar-mode

and peform a redisplay.  Things like `indent-buffer' in foo-mode would
use `inhibit-point-motion-hooks' to avoid switching to another mode
intermittently.  In any case, the original philosophy that "each buffer
has only one major mode at a time" would remain intact.

AFAICT only three major modes - cc-awk, cperl, and perl - currently use
generic strings for their purposes.  Whether and how to accomodate them
is beyond my comprehension.

Just an idea - with blatant disregard for the particular needs of PHP
and friends.