I believe this is the correct result as $a is not an instance of the trait but rather of the class that utilizes the trait.

Mike: That's one way of looking at it. My point of view is that a trait adds methods to class' interface (where interface is a set of public members of a class - both methods and fields) and this should be reflected in class type.

Currently (unless I missed something that was not mentioned in RFC - I admit to not have gone through SVN version of docs) there seem to be no way to check if class uses a trait or no. Only new functions mentioned are trait_exists() and get_declared_traits(). Checking if the object has a method will not work, if trait is meant to override method in host class. Having traits reflected in object's type, would solve this problem nicely. Adding another function to glabal namespace (uses_trait() ?) is not something I would like to see.

> $a is not an instance of the trait but rather of
> the class that utilizes the trait.

You can say the same of interfaces and abstracts, but is_a returns true for 
them.

Test script:
---------------
trait someTrait {}

interface someInterface {}

abstract class someAbstract {}

class someClass extends someAbstract implements someInterface {
    use someTrait;
}

$a = new someClass();
var_dump(is_a($a, 'someClass'));
var_dump(is_a($a, 'someAbstract'));
var_dump(is_a($a, 'someInterface'));
var_dump(is_a($a, 'someTrait'));

Expected result:
----------------
bool(true)
bool(true)
bool(true)
bool(?)

Actual result:
----------------
bool(true)
bool(true)
bool(true)
bool(false)

My view on this issue is that traits do not constitute types and do not provide interfaces. (I proposed the later one in the beginning in the sense of traits are interfaces with implementation, but this was disregarded to make clear that traits are neither classes nor interfaces.)

Thus, is_a should also not say anything about a trait.
Traits are not units of encapsulation, they do not guarantee to provide/protect any invariants.

However, if you need to know what traits are used by a class, please refer to: ReflectionClass::getTraits()

I just noticed that we have the following function in the SPL:
http://php.net/manual/en/function.class-implements.php
That should probably be mirrored to provide the same functionality as ReflectionClass::getTraits().

Not sure what the design policies are here. From a symmetry perspective there should be a class_uses() function, but from my personal perspective, class_implements should get nuked and uses should transition to the reflection extension if they need such meta programming facilities. Well, the later is not practical, so we will probably need to have class_uses().

Added class_uses(..) in SPL to mirror existing class_implements(..) per http://svn.php.net/viewvc?view=revision&revision=313640

This bug has been fixed in SVN.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.