Opposite of array_intersect?

That sounds like a job for array_diff.

Returns an array containing all the entries from array1 that are not present in any of the other arrays.

array_diff is definitely the obvious choice but it is not technically the opposite of array interesect. Take this example:

$arr1 = array('rabbit','cat','dog');

$arr2 = array('cat','dog','bird');

print_r( array_diff($arr1, $arr2) );

What you want is a result with 'rabbit' and 'bird' in it but what you get is only rabbit because it is looking for what is in the first array but not the second (and not vice versa). to truly get the result you want you must do something like this:

$arr1 = array('rabbit','cat','dog');

$arr2 = array('cat','dog','bird');

$diff1 = array_diff($arr1, $arr2);
$diff2 = array_diff($arr2, $arr1);
print_r( array_merge($diff1, $diff2) );

Note: This method will only work on arrays with numeric keys.

$diff = array_diff($array1, $array2);

array_diff()

Just to clarify as I was looking into this question the answers of @Jon and @Dallas Caley are both correct depending on the domain of your arrays.

If the array against what you are comparing is the full domain of your results then a simple array_diff will suffice as per @Jon answer.

If the array against what you are comparing is NOT the full domain of your results then you should go with the double array_diff as per @Dallas Caley answer.