flatten nested array, where each level of the the tree represents a different category to search by, and preserve traversal metadata.

gistfile1.php

/** 
* @keys     n-length list of "labels", where the @key[i] is the category label of the ith level of @dat
* @dat      a nested associative array, tree-like, with n+1 levels, where 1..n levels of the tree represent categories
*           and the n+1 level is the actual request data. 
* returns:  list of associative arrays, each representing rows to be inserted into database.
*
*   flattens nested array into a list of "rows", where the |rows| == |leaves in tree|.
*   each "row" contains the data within the leaves, plus n additional key-value pairs represented by
*   label => category (i.e. [...country => 'US', type => 'tablet'] when @keys = ['country', 'type'], and @dat has n+1==3 levels.)
**/

public static function nestedArrayToListOfArrays($keys, $dat) {
    if (!isset($keys)) {
        return $dat;
    }
    $data = array();
    array_push($data, array(
        'data' => $dat //$this->to_array()
    ));
    while (count($keys)>0) {
        $new_data = [];
        $key = array_shift($keys);
        foreach($data as $num => $row) {
            foreach($row['data'] as $k => $v) {
                $new_row = $row;
                $new_row[$key] = $k;
                $new_row['data'] = $v;
                array_push($new_data, $new_row);
            }
        $data = $new_data;
        }
    }
    return $data;
}