Compatibility is being tracked in compatibility.md. We need to aim for all features it is possible to implement. Unfortunately we cannot implement "all" the functionality, for example:

• Extension-related things ReflectionExtension - I don't think we have a way to access the information required by this, nor to tie it all up with our userland reflection (I may be wrong?) - see Implement extensions reflection #15
• Methods ReflectionMethod->invoke as we are not actually loading the classes, we cannot really invoke methods (yes, it's technically possible, but not logical) - see Implement accessors without instantiating #14
• State modifications like ReflectionProperty->setAccessible and ReflectionProperty->setValue these require instances of classes, but we never instantiate classes - see Implement accessors without instantiating #14
• Instance-requiring ReflectionObject simply because it requires an instance, we cannot implement it, because that would require the class to be loaded - see Implement ReflectionObject #16
• ReflectionFunctionAbstract->isClosure can't be implemented currently - see Add ability to create a ReflectionFunction from a closure #37
• Internal reflection is not currently possible - see Create ReflectionFunctions/ReflectionClasses from internal definitions #38

Compatibility summary

• ReflectionClass - PR Compatibility for ReflectionClass #59
• ReflectionFunctionAbstract - PR Compatibility for ReflectionFunctionAbstract and ReflectionFunction #40
• ReflectionMethod - PR Pretty much compatible ReflectionMethod #58
• ReflectionParameter - PR Adds compatibility for ReflectionProperty #56
• ReflectionFunction - PR Compatibility for ReflectionFunctionAbstract and ReflectionFunction #40
• ReflectionProperty - PR Compatibility for ReflectionParameter #9