Q:

Foreach not working - PHP

I need to print the below array structure as:
Node Title 1

  topic 1

  topic 2

  topic 3

  topic 4

    asset title1 

    asset title2

    asset title3

How can i do using foreach - PHP
What i have done is :
foreach($output['fields'] as $key => $value) {
        if($key == 'title') {
            echo $value;
        }
        if(count($value['main_topic'])) {
            foreach($value['main_topic'] AS $mainkey => $main_topic) {
                echo $main_topic['topic_title'];
            }
        }
    }

The above syntax is printing the title. But not the main_topic array.
Array
(
    [fields] => Array
        (
            [nid] => 136
            [node_title] => Node title 1
            [node_type] => curriculum
            [title] => Node title 1
            [main_topic] => Array
                (
                    [0] => Array
                        (
                            [row_id] => 136
                            [topic_id] => 411847
                            [weight] => 10
                            [topic_title] => topic 1
                        )

                    [1] => Array
                        (
                            [row_id] => 136
                            [topic_id] => 411839
                            [weight] => 2
                            [topic_title] => topic 2
                        )

                    [2] => Array
                        (
                            [row_id] => 136
                            [topic_id] => 411840
                            [weight] => 3
                            [topic_title] => topic 3
                        )

                    [3] => Array
                        (
                            [row_id] => 136
                            [topic_id] => 411841
                            [weight] => 4
                            [topic_title] => topic 4
                            [subfield] => Array
                                (
                                    [1] => Array
                                        (
                                            [asset_title] => asset title 1
                                        )

                                    [2] => Array
                                        (
                                            [asset_title] => asset title 2
                                        )

                                    [3] => Array
                                        (
                                            [asset_title] => asset title 3
                                        )

                                )

                        )

                )

        )

)

A:

That is because you are iterating over all $output['fields']. 
There will never be a $value with key 'main_topic' because the key 'main_topic' is contained in the $output['fields'] array and thus exists only as $key in your foreach. The array you want is $value
Your code should be like:
foreach($output['fields'] as $key => $value) {
        if($key == 'title') {
            echo $value;
            continue;
        }
        if($key == 'main_topic' && is_array($value)) {
            foreach($value as $main_topic) {
                echo $main_topic['topic_title'];
            }
        }
    }

To complete this answer with a full solution (including asset titles), below is how I would write it. 
Because $output['fields'] is the starting point and to make the code more readable, I create a reference to the starting node using the =& operator so the array is not copied in memory. I do the same with the inner foreachs. Since we are not modifying data, referencing the variables is sufficient and consumes less memory and CPU:
if (is_array($output['fields'])) {
    $node =& $output['fields'];
    echo $node['title'];
    if(is_array($node['main_topic'])) {
        foreach($node['main_topic'] as &$main) {
            echo $main['topic_title'];
            if(is_array($main['subfield'])) {
                foreach($main['subfield'] as &$asset) {
                    echo $asset['asset_title'];
                }
            }
        }
    } 
}
else {
    echo "no menu";
}

