Fix phpstan/phpstan#7088: Accessing SimpleXMLElement nodes using braces gives mixed type

[phpstan-bot]

Summary

Accessing SimpleXMLElement properties using dynamic names ($prop->{$x}) was incorrectly resolved as mixed instead of SimpleXMLElement|null. Static property names ($prop->foo) and constant string expressions ($prop->{'foo-bar'}) already worked correctly.

Changes

• Added fallback logic in src/Analyser/ExprHandler/PropertyFetchHandler.php for non-constant string property names: when the name is a string type but not a constant string, query the var type's property reflection to determine the type
• New regression test in tests/PHPStan/Analyser/nsrt/bug-7088.php

Root cause

In PropertyFetchHandler::resolveType(), when the property name expression is not an Identifier and not a constant string, the code immediately returned MixedType. For classes like SimpleXMLElement that have a PropertiesClassReflectionExtension accepting any property name, we can still determine the correct type by querying the property reflection with a representative property name.

Test

Added tests/PHPStan/Analyser/nsrt/bug-7088.php that verifies all three property access patterns on SimpleXMLElement resolve to (SimpleXMLElement|null):

• $prop->foo (static name)
• $prop->{'foo-bar'} (constant string expression)
• $prop->{$x} (dynamic variable name)

Fixes phpstan/phpstan#7088

[VincentLanglet]

What about passing null as propertyName rather than a custom constant __phpstan_dynamic_property

Edit: The method in Type interface requires a string

[VincentLanglet]

While using a hardcoded constant __phpstan_dynamic_property is not the best, I don't see another easy way without BC break (and lot of changes) just to have hasInstanceProperty() method accepting NULL.

I think it's good enough but we might prefer ask validation on ondrej. WDYT ?