Trying to learn about php's arrays today.

I have a set of arrays like this:

$a = array ( 
    0 => array ( 'value' => 'America', ), 
    1 => array ( 'value' => 'England', ),  
    2 => array ( 'value' => 'Australia', ), 
)
$b = array ( 
    0 => array ( 'value' => 'America', ), 
    1 => array ( 'value' => 'England', ), 
    2 => array ( 'value' => 'Canada', ), 
)

I need to get the 'new' subarrays that array b brings to the table.

ie, I need to return array ( 'value' => 'Canada', )

I thought I could first merge $a+$b and then compare the result to $a.

$merge = array_merge($a,$b);
$result = array_diff($merge,$a);

But somehow that does not work. It returns array()

How come? And thanks!