i have an multi dimensional array that's grouped. But i would like to sum up amounts and have totals for each group within the array itself. but i am struggling with summing up the amounts for each group in the array
Here is how my array looks like

```
Array
        (
            ['Network 1'] => Array
                (
                    [0] => Array
                        (
                            [MSISDN] => 27729554427
                            [Network] => 'Network 1'
                            [Date] => '12-Mar-2016'
                            [Product] => 'Loan Product 1'
                            [Amount] => 1000.00
                        )

                    [1] => Array
                        (
                            [MSISDN] => 27725326345
                            [Network] => 'Network 1'
                            [Date] => '18-Mar-2016'
                            [Product] => 'Loan Product 2'
                            [Amount] => 3000.00
                        )
                )

            ['Network 2'] => Array
                (
                    [0] => Array
                        (
                            [MSISDN] => 27722342551
                            [Network] => 'Network 2'
                            [Date] => '16-Mar-2016'
                            [Product] => 'Loan Product 1'
                            [Amount] => 500.00
                        )

                    [1] => Array
                        (
                            [MSISDN] => 27729234533
                            [Network] => 'Network 2'
                            [Date] => '01-Apr-2016'
                            [Product] => 'Loan Product 1'
                            [Amount] => 100.00
                        )
                )
        )

```

this is what i want

```
Array
    (
        ['Network 1'] => Array
            (
                [0] => Array
                    (
                        [MSISDN] => 27729554427
                        [Network] => 'Network 1'
                        [Date] => '12-Mar-2016'
                        [Product] => 'Loan Product 1'
                        [Amount] => 1000.00
                    )

                [1] => Array
                    (
                        [MSISDN] => 27725326345
                        [Network] => 'Network 1'
                        [Date] => '18-Mar-2016'
                        [Product] => 'Loan Product 2'
                        [Amount] => 3000.00
                    )
            )
        ['Network 1 Aount total'] => 4000.00

        ['Network 2'] => Array
            (
                [0] => Array
                    (
                        [MSISDN] => 27722342551
                        [Network] => 'Network 2'
                        [Date] => '16-Mar-2016'
                        [Product] => 'Loan Product 1'
                        [Amount] => 500.00
                    )

                [1] => Array
                    (
                        [MSISDN] => 27729234533
                        [Network] => 'Network 2'
                        [Date] => '01-Apr-2016'
                        [Product] => 'Loan Product 1'
                        [Amount] => 100.00
                    )
            )
        ['Network 2 Aount total'] => 600.00    
    )

```

this is what i tried 

```
        function _group_by($array, $key) {
        $return = array();
        foreach($array as $val) {
            $return[$val[$key]][] = $val;
            //$return[$val['Amount']] += $val; // this line is failing the one failing
        }

            echo '<pre>';
            print_r($return);
            echo '</pre>';
        exit();
        return $return;
    }

```