php json_encode and decode multi-dimension array

php json_encode and decode multi-dimension array

I have a php array structure
Array
(
[car] => Array
(
[red] => 0.333
[orange] => 0.333
[blue] => 0.333
)
[truck] => Array
(
[white] => 0.333
[green] => 0.333
[blue] => 0.333
)
)
I have been using serialize to save the array to a text file, and
unserialize to get back the array form. Unfortunately, the serialized
array is getting very large, but mostly due to the float point (bug or by
design) conversion when serialized. For example, instead of 0.333, the
serialized process converts .333 into
.3333333333333333333333333333333333333333333333333. This made me want to
switch to json_encode to save the array. When compare serialize to
json_encode, the serialized file is 40MB in size vs 8MB size for
json_encode.
That is great, except when I try to json_decode the file, it is no longer
in the array form. I tried json_decode($array, true), but that does not
work either.
Any idea how to get json_encode to work for this example?
TIA
PS, my floating point number was generated by rounding off the decimals.
Another answer that I found on StackO suggested that instead of using
round($part/$sum, 3);, use sprintf('%.3f', $part/$sum); which turned the
floating point into a string. That alone cut the serialized file down from
40MB to 19MB, but it still much larger than the json_encode file size of
8MB.