> Hope display: 5.00

Why?  JSON_PRESERVE_ZERO_FRACTION is meant to enforce floating
point representation (i.e. not integer); isn't that sufficient
for data exchange?

$arr = [
   'a' => (float)1.00
 ];
echo json_encode($arr,JSON_PRESERVE_ZERO_FRACTION);

output:{"a":1.0}

I want output: {"a":1.00}

But why would you want that output?  For pretty printing?

Yes, Business needs.

I'm thinking, should add another one, JSON_PRESERVE_DOUBLE_ZERO_FRACTION
^_^

There is no way for your example to give the output you want. When you write "$a = 5.00;", PHP doesn't store anywhere the fact that you typed two zeroes, it just stores the number 5 in a floating point variable. Similarly, when you call json_encode([5.00]), PHP is just passing the floating point number 5 to the function json_encode. There is no way for the function to know that you wanted two trailing zeroes on this particular 5, but one or three or nineteen trailing zeroes on other 5s.

A JSON_PRESERVE_DOUBLE_ZERO_FRACTION constant would at least be possible, but it seems oddly specific - why not also JSON_PRESERVE_TRIPLE_ZERO_FRACTION, JSON_PRESERVE_QUADRUPLE_ZERO_FRACTION, and so on? 

Note that adding a single zero is not just a matter of presentation - other code parsing the resulting JSON can see that the value "5.0" was intended as a floating point number, and "5" as an integer. The difference between "5.00" and "5.0", however, would only matter to human readers.

No matter how the bottom layer is designed, but I have encountered this problem in actual development, it is necessary for business, enter 5.00, json_enocde shows 5.00

If you need to preserve the exact formatting of the number, store it as a string.

Alternatively and more likely what you want to do, format it with two zeros when printing.