PHP- Multidimensional array, key 0

I have an array like:

 $a = array(array('Hello','abc'), 'd');
 var_dump($a);

    array (size=2)
  0 =>
    array (size=2)
      'title' => string 'Hello' (length=40)
      'class' => string 'abc' (length=3)
  1 => string 'd' (length=6)

var_dump($a[0]);
 array (size=2)
          'title' => string 'Hello' (length=40)
          'class' => string 'abc' (length=3)

Why $a[0] is not empty? Anybody can point this out?

//Edit

I've tested that key 0 with this code:

If(empty($a[0]))
{
    // Do A
}
else
{
    // Do B
}

Why this return Do B?

---

This array $a = array(array('Hello','abc'), 'd'); is equivalent to:

 $a = array(0 => array(0 => 'Hello', 1 => 'abc'), 1 => 'd');

PHP will substitute those keys.