Get bilateral differences between two flat arrays

So here is my code from my genetics calculator:

// Set up male dominant traits
$maleDom = 'WT';
// Set up male recessive traits
$maleRec = 'ba,b,ec';
// Set up male snow traits
$maleSnow = '';

// Set up female dominant traits
$femaleDom = 'WT';
// Set up female recessive traits
$femaleRec = 'ba,b,mp';
// Set up female snow traits
$femaleSnow = '';

$maleRecArray = explode(',', $maleRec);
$femaleRecArray = explode(',', $femaleRec);

$nonMatchedRecessives = array_diff($maleRecArray, $femaleRecArray);

Which throws out Array ( [2] => ec ). But you can see that ec and mp exist in separate arrays. So surely it should be showing mp too as it's different to the other array?

You need to run it twice, comparing the 2 arrays in reverse order, like this:

array_merge(array_diff($maleRecArray, $femaleRecArray), array_diff($femaleRecArray, $maleRecArray));

Which spits out:

Array
(
    [0] => ec
    [1] => mp
)

Example