php getting unique values of a multidimensional array

Possible Duplicate:
php multi-dimensional array remove duplicate

I have an array like this:

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

How can I remove the duplicate values so that I get this:

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

I tried using array_unique, but that doesn't work due to this array being multidimensional, I think.

Edit: I also need this array to be multi-dimensional and in this format, I can't flatten it.