How can I extract values from an array (array_values isn’t working)?

I have the following PHP array

 [1] => Array
                                (
                                    [0] => 9780881220766
                                    [1] => 0881220760
                                )

                            [2] => Array
                                (
                                    [0] => 9780141374284
                                    [1] => 0141374284
                                )

                            [3] => Array
                                (
                                    [0] => 057305133X
                                    [1] => 9780573051333
                                ))

I would like the output to be as follows:

  [0] => 9780881220766
                                        [1] => 0881220760
                                        [2] => 9780141374284
                                        [3] => 0141374284
                                        [4] => 057305133X
                                        [5] => 9780573051333

I tried using array_values but I’m not getting any output.

I also tried to extract the values using a foreach loop:

   foreach ($rawIsbns as $isbnKey => $isbnValue){
            $formattedIsbns[] = $isbnValue;
        }

However, I’m not managing.

Answer

I managed to solve the problem by initializing an array and adding values of subsequent nested arrays using array_push

function isbns($rawIsbns) {
        $prevArrayValues = null;
        foreach ($rawIsbns as $value) {
            if (!isset($prevArrayValues)) {
                $prevArrayValues = []; //initiallise first value unless initialized
            }

            if (empty($value)) {
                $return = []; //disregard empty values
            } else {
                $return = $value; // add arrays to existing array
            }

            $prevArrayValues = array_merge($prevArrayValues, $return);
        }
        return $prevArrayValues;
    }