How to check if a value is in a 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?