Review the usage of apostrophes in error messages

Closes GH-5590

Improve error messages for invalid property access

Closes GH-5446
Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>

Improve undefined variable error messages

Closes GH-5312

Revert "Fetch for read in nested property assignments"

This reverts commit bb43a3822e42dbd15b1d416a166549d3980b379a.

After thinking about this a bit more, this is now going to be

Revert "Fetch for read in nested property assignments"

This reverts commit bb43a3822e42dbd15b1d416a166549d3980b379a.

After thinking about this a bit more, this is now going to be
a complete solution for the "readonly properties" case, for example:

unset($foo->readOnly->bar);

should also be legal and

$foo->readOnly['bar'] = 42;

should also be legal if $foo->readOnly is not an array but an
ArrayAccess object.

I think it may be better to distinguish better on the BP_VAR flag
level. Reverting for now.

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Fetch for read in nested property assignments

$a->b->c = 'd';

is now compiled the same way as

$b = $a->b;
$b->c = 'd';

That is, we perform a read f

Fetch for read in nested property assignments

$a->b->c = 'd';

is now compiled the same way as

$b = $a->b;
$b->c = 'd';

That is, we perform a read fetch on $a->b, rather than a write
fetch.

This is possible, because PHP 8 removed auto-vivification support
for objects, so $a->b->c = 'd' may no longer modify $a->b proper
(i.e. not counting interior mutability of the object).

Closes GH-5250.

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Convert some notices to warnings

Part of https://wiki.php.net/rfc/engine_warnings.

Throw Error when writing property of non-object

This removes object auto-vivification support.

This also means that we can remove the corresponding special
handling for typed pr

Throw Error when writing property of non-object

This removes object auto-vivification support.

This also means that we can remove the corresponding special
handling for typed properites: We no longer need to check that a
property is convertible to stdClass if such a conversion might
take place indirectly due to a nested property write.

Additionally OBJ_W style operations now no longer modify the
object operand, and as such we no longer need to treat op1 as a
def in SSA form.

The next step would be to actually compile the whole LHS of OBJ_W
operations in R rather than W mode, but that causes issues with
SimpleXML, whose object handlers depend on the current compilation
structure.

Part of https://wiki.php.net/rfc/engine_warnings.

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Fixed bug #75921

Ensure that the "creating default object from empty value" warning is
always thrown. Previously some cases were missing the warning, in
particular those going throug

Fixed bug #75921

Ensure that the "creating default object from empty value" warning is
always thrown. Previously some cases were missing the warning, in
particular those going through FETCH_OBJ_W rather than a dedicated
opcode (like ASSIGN_OBJ).

One slightly unfortunate side-effect of this change is that something
like $a->b->c = 'd' will now generate two warnings rather than one
when $a is null (one for property b, one for property c).

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