Why the foreach array is not executing as expected and not generating the desired output? What mistake I'm making?

Question

I've a big array titled $allFeeds as follows :

Array
(
[custom_data_cache] => Array
        (
[answer] => Array
                (
                    [0] => Array
                        (
                            [answer_id] => 289
                            [poll_id] => 115
                            [answer] => Let's meet now
                            [total_votes] => 0
                            [ordering] => 1
                            [vote_percentage] => 0
                        )

                    [1] => Array
                        (
                            [answer_id] => 290
                            [poll_id] => 115
                            [answer] => Let's plan sometime later
                            [total_votes] => 0
                            [ordering] => 2
                            [vote_percentage] => 0
                        )

                )

        )
)

I want to make value in inner most ['answer'] key, not the outer ['answer'] key blank(In above array two such elements exist) but I'm not able to do it.

Following is the code I tried but the array is not changing at all.

foreach ($allFeeds['custom_data_cache']['answer'] as $key => $value) {
      $key[$value]['answer'] = '';          
    }
print_r($allFeeds);

Again the same array prints. Expected output is as follows :

Array
(
[custom_data_cache] => Array
        (
[answer] => Array
                (
                    [0] => Array
                        (
                            [answer_id] => 289
                            [poll_id] => 115
                            [answer] => 
                            [total_votes] => 0
                            [ordering] => 1
                            [vote_percentage] => 0
                        )

                    [1] => Array
                        (
                            [answer_id] => 290
                            [poll_id] => 115
                            [answer] => 
                            [total_votes] => 0
                            [ordering] => 2
                            [vote_percentage] => 0
                        )

                )

        )
)

Please help me by correcting the mistake I'm making in array manipulation.

Answer 1

Your local (for the loop) variable $key hold just the index and not a reference to the element (array) under that index and $value holds a copy of that element (array). The assignment $key[$value]['answer'] = ''; is wrong on so many levels (syntactically as well as semantically). Do you have your error_reporting turned off by the way?
Thus change

foreach ($allFeeds['custom_data_cache']['answer'] as $key => $value) {
    $key[$value]['answer'] = '';          
}

to

foreach ($allFeeds['custom_data_cache']['answer'] as $key => $value) {
    $allFeeds['custom_data_cache']['answer'][$key]['answer'] = '';          
}

Answer 2

You can easily set inner most answer values to empty string like so:

foreach ($allFeeds['custom_data_cache']['answer'] as &$value) {
  $value['answer'] = '';          
}
print_r($allFeeds);

Using &value allows you to modify the array without retraversing all dimensions.