create an array that can be passed to `CURLOPT_POSTFIELDS` that contains mutidimension arrays and `CURLFile`s

build_postfields.php

<?php
/**
 * Use this to send data with multidimensional arrays and CURLFiles
 *  `curl_setopt($ch, CURLOPT_POSTFIELDS, build_post_fields($postfields));`
 *
 * @param        $data
 * @param string $existingKeys - will set the paramater name, probably don't want to use
 * @param array  $returnArray - Can pass data to start with, only put good data here
 *
 * @return array
 * @author Yisrael Dov Lebow <lebow@lebowtech.com>
 * @see https://stackoverflow.com/questions/3453353/how-to-upload-files-multipart-form-data-with-multidimensional-postfields-using
 * @see http://stackoverflow.com/questions/35000754/array-2-string-conversion-while-using-curlopt-postfields/35002423#comment69460359_35002423
 */
function build_post_fields( $data,$existingKeys='',&$returnArray=[]){
    if(($data instanceof CURLFile) or !(is_array($data) or is_object($data))){
        $returnArray[$existingKeys]=$data;
        return $returnArray;
    }
    else{
        foreach ($data as $key => $item) {
            build_post_fields($item,$existingKeys?$existingKeys."[$key]":$key,$returnArray);
        }
        return $returnArray;
    }
}