Description:
------------
It is impossible to override an abstract method with another abstract method.
This is useful if you are trying to indicate (to both IDE and other developers) 
that the overriden method returns type is expected to be a subclass of the 
inherited method return type.

PS. Java allows it and it is generally a more restrictive language than PHP.

Test script:
---------------
abstract class RuntimeExceptionReporter extends ExceptionReporter {

    /**
     * @return RuntimeException
     */
    public abstract function getExceptionToReport();

    // ... some operations specific to RuntimeException

}

abstract class ExceptionReporter {

    /**
     * @return Exception
     */
    public abstract function getExceptionToReport();

    // ... some operations on an Exception

}

Expected result:
----------------
Correct inheritance

Actual result:
--------------
Fatal error: Can't inherit abstract function 
ExceptionReporter::getExceptionToReport() (previously declared abstract in 
RuntimeExceptionReporter) in xxx on line yyy