将两级多维数组保存到php中的csv文件中

Question

I have an array that when printed with print_r looks like this:

Array(
[0] => Color
[Color] => Array
    (
        [0] => Red
        [1] => white
        [2] => Bleu
     )
[1] => size
[size] => Array
    (
        [0] => Small 
        [1] => Med 
        [2] => Large
     )
)

I am trying to dump it to a csv file that should look like this when finished:

Color: Red, White, Blue

size: Small, "1-1/2 Inch", "8 inch"

The code below gets part of the job done but I am having trouble getting the array keys to print out at the beginning of each line in the csv file. Thanks.

<?php

print_r($mf_url_array);

$fp = fopen('file.csv', 'w');
fputcsv($fp, array_keys($mf_url_array));
foreach ($mf_url_array as $fields) {
    if(!empty($fields) and is_array($fields)){
    fputcsv($fp, $fields);
    }
}

fclose($fp);
?>