[DirkHoffman]

Fixes GH-24250.

LLM-generated description: Since 94136cf (GH-22709), the ZEND_ASSIGN_OBJ fast path skips the set visibility check when the cache slot is primed, relying on zend_std_write_property() to reset the slot when the check fails. When an unset property is written through __set(), the check is skipped altogether and the slot primed by zend_get_property_offset() stays valid, so the next write at the same call site to an initialized property bypasses protected(set)/private(set) (also from child classes, and for private(set) readonly in __clone()). This resets the cache slot in that case as well, so __set() forwarding itself is unchanged.

The new test fails on 8.6.0RC3 / current PHP-8.6 and passes on 8.4 and 8.5 (same expected output).

Found and verified with the help of AI tools.

[anun333]

Quoted below: written with Claude, an LLM, as CONTRIBUTING.md asks us to mark. The builds and test runs behind it are on my machine; the commands to repeat them are under the fold.

Tested this alongside #24566, which fixes the same GH-22709 regression (94136cf9bfb, on PHP-8.6 and master only), each applied onto today's PHP-8.6 (a588565de24), debug builds:

Test	PHP-8.6	this PR	#24566
gh24250.phpt (this PR)	FAIL	PASS	PASS
cache_slot_magic_set.phpt (#24566)	FAIL	PASS	PASS
cache_slot_clone_reinit.phpt (#24566)	FAIL	FAIL	PASS

The third is the other path #24566 covers: a "Cannot modify readonly property" error also returns with the cache slot primed, so the same $p->x = $v opline later reinitializes a public readonly property from global scope inside __clone(). With this PR it prints int(3) where "Cannot modify protected(set) readonly property P::$x from global scope" is expected. Zend/tests/asymmetric_visibility, readonly_props and property_hooks pass on all three builds.

How this was tested

Linux x86_64, gcc 13.3. Each tree is origin/PHP-8.6 with the PR's commit cherry-picked on top:

./buildconf --force
./configure --enable-debug --disable-cgi --disable-phpdbg --disable-fileinfo --enable-pcntl --enable-zend-test --enable-fpm
make -j4

The three tests copied out of the two PRs and run on each build with TEST_PHP_EXECUTABLE=sapi/cli/php sapi/cli/php run-tests.php -n -q <tests>; the folders with run-tests.php -n -q Zend/tests/asymmetric_visibility Zend/tests/readonly_props Zend/tests/property_hooks.

[ArtUkrainskiy]

Reproduced on current master without the patch. Once a call site is routed through __set, a later protected(set) write from global scope goes through silently instead of throwing. The slot reset mirrors the existing error path and stays narrow enough to leave normal writes alone. Test covers the child-class and readonly-clone cases too. LGTM.