Stdlib: SimpleXMLElement::xpath() invalid expression → warning + false (#22720)

[PurHur]

Summary

• Match php-src zim_simplexmlelement_xpath / xmlXPathEvalExpression failure: invalid XPath → E_WARNING SimpleXMLElement::xpath(): Invalid expression + false (not []).
• Valid empty-result queries still return [].
• User-script AOT: register SimpleXMLElement::xpath proxy; compile-time tree + literal path lower via host SimpleXML (false / empty node-set).

php-src: ext/simplexml/simplexml.c (sxe_xpath)
PHP impl: ext/simplexml/VmSimpleXml.php, JitSimpleXmlUserScript::tryXpath, JitSimpleXmlXpath
Closes #22720

Test plan

• VM: php bin/vm.php test/repro/issue_22720_simplexml_xpath_invalid.php → warning + false / boolean; empty query → array
• JIT: php bin/jit.php test/repro/issue_22720_simplexml_xpath_invalid.php (same)
• AOT: php bin/compile.php -o /tmp/sxe22720 test/repro/issue_22720_simplexml_xpath_invalid_aot.php && /tmp/sxe22720 → warning + false / boolean / array:0
• ./script/phpunit.sh --filter 'xpath_invalid_expression|xpath_child_and_absolute|xpath_register_namespace|xpath_live' → OK (8)
• php script/bootstrap-inventory.php --check → OK 6505/6505
• php script/check-selfhost-spine-coverage-sync.php → OK

Verification transcript (Docker)

=== AOT ===
PHP Warning:  SimpleXMLElement::xpath(): Invalid expression in  on line 0
false
boolean
array:0
EXIT:0
=== VM ===
PHP Warning:  SimpleXMLElement::xpath(): Invalid expression ...
false
boolean
array (
)
array

Made with Cursor

[PurHur]

Cleanup bot: deleted leftover remote branch fix/22720-simplexml-xpath-invalid-expression (squash-merged here; #22720 closed; GD withhold commit also on master via #22757).