PHP Array_diff when there are duplicate array values

Question

I have two arrays containing repeating values:

$test1 = Array(
        "blah1",
        "blah1",
        "blah1",
        "blah1",
        "blah2"
    );

    $test2 = Array(
        "blah1",
        "blah1",
        "blah1",
        "blah2"
    );

I am trying to get array difference:

$result = array_diff($test1,$test2);

echo "<pre>";
print_r($result);

I need it to return array with single value blah1, yet it returns empty array instead...

I suspect it has something to do with fact there are duplicate values in both arrays, but not sure how to fix it...

Please help!!

EDIT:

End up writing this function to do the trick:

function subtract_array($array1,$array2){
    foreach ($array2 as $item) {
        $key = array_search($item, $array1);
        unset($array1[$key]);
    }
    return array_values($array1);
}

Answer 1

array_diff compares the first array to the other array(s) passed as parameter(s) and returns an array, containing all the elements present in the first array that are not present in any other arrays. Since $test1 and $test2 both contain "blah1" and "blah2", and no other values, actually, the expected behavior of array_diff is the one that you have experienced, that is, to return an empty array, since, there is no element in $test1 which is not present in $test2.

Further read. Also, read some theory to understand what you are working with.

Answer 2

Spotted a problem with Acidon's own solution. The problem comes from the fact that unset($array[false]) will actually unset $array[0], so there needs to be an explicit check for false (as David Rodrigues pointed out as well.)

function subtract_array($array1,$array2){
    foreach ($array2 as $item) {
        $key = array_search($item, $array1);
        if ( $key !== false ) {
            unset($array1[$key]);
        }
    }
    return array_values($array1);
}

Some examples

subtract_array([1,1,1,2,3],[1,2]);            // [1,1,3]
subtract_array([1,2,3],[4,5,6]);              // [1,2,3]
subtract_array([1,2,1],[1,1,2]);              // []
subtract_array([1,2,3],[]);                   // [1,2,3]
subtract_array([],[1,1]);                     // []
subtract_array(['hi','bye'], ['bye', 'bye']); // ['hi']