wtf.md

Good:

interface MyInterface
{
  function foo();
}

class MyClass implements MyInterface
{
  public function foo($a = null, $b = null)
  {
    echo $a + $b;
  }
}

$class = new MyClass();
$class->foo(2, 2);

Result:

4

Bad:

interface MyInterface
{
  function foo();
}

class MyClass implements MyInterface
{
  public function foo($a, $b)
  {
    echo $a + $b;
  }
}

$class = new MyClass();
$class->foo(2, 2);

Result:

Fatal error: Declaration of MyClass::foo() must be compatible with MyInterface::foo()