PHP: What does this error mean? Abstract method must be compatible?

Question

I have an abstract class. I'm extending that class. I'm getting this error:

Fatal error: Declaration of Default_Model_FoobarMapper::_setClassVarsFromRow() must be compatible with that of Default_Model_AbstractMapper::_setClassVarsFromRow() in /location/to/models/FoobarMapper.php on line 3

What does this usually mean?

Update: Found out that my type hinting was throwing an error. You can't do this:

abstract class MyAbstractClass
{
    abstract protected function _myFunction($array, $generic_class);
}

class Foobar extends MyAbstractClass
{
    protected function _myFunction($array, Specific_Class $specific_class)
    {
        //etc.
    }
}

Answer 1

The arguments you declare for _setClassVarsFromRow() must be identical to those in the abstract.

For example, if your abstract says

function _setClassVarsFromRow($arg1, $arg2 = null)

you can't implement

function _setClassVarsFromRow($arg1, $arg2, $arg3 = null)