in_array() and multidimensional array

I use in_array() to check whether a value exists in an array like below,

$a = array("Mac", "NT", "Irix", "Linux");
if (in_array("Irix", $a)) 
{
    echo "Got Irix";
}

//print_r($a);

but what about an multidimensional array (below) – how can I check that value whether it exists in the multi-array?

$b = array(array("Mac", "NT"), array("Irix", "Linux"));

print_r($b);

or I shouldn’t be using in_array() when comes to the multidimensional array?