Description:
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The status of the flag ZEND_ACC_IMPLICIT_PUBLIC is somewhat unclear. In the ZE:

* PHP 5.3 checks for it in a few places, but doesn't set it anymore as the body of zend_do_declare_implicit_property is commented out.
* trunk retains only the definition of ZEND_ACC_IMPLICIT_PUBLIC, everything else is gone.

So it seems that the flag ZEND_ACC_IMPLICIT_PUBLIC, apparently originally used to signal dynamic properties, is only used by the reflection extension. The reflection extension, not being able to depend on the Zend Engine to set it, sets the flag itself when it finds a dynamic property, i.e. a property that is in the object hash table but has no property_info (ReflectionProperty::__construct and ReflectionClass::getProperty).

This would be unnecessary if zend_get_property_info set the flag ZEND_ACC_IMPLICIT_PUBLIC over here:

http://lxr.php.net/opengrok/xref/PHP_TRUNK/Zend/zend_object_handlers.c#310

In that case, the reflection extension could just call zend_get_property_info and not do directly lookups in ce->properties_info. Though right now it would be possible to call zend_get_property_info and check for equality to &EG(property_info), this is suboptimal.