Description:
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The json_decode() function improperly and incosistently decodes JSON 
objects that have an empty string as one of their object keys/property 
names. In some cases the server crashes, some cases work properly, and 
other cases return "_empty_" as the array key.

The "_empty_" key and crashes are show stoppers for me.

Having an empty object key/property such as {"": "value"} is totally 
valid, and even common, JSON.

As a side note, I'll open a separate bug for this, json_encode(array
(""=>"value")) does not work either.

Reproduce code:
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1) var_dump(json_decode('{"":"value"}', true));
2) var_dump(json_decode('{"":"value", "key":"value"}', true));
3) var_dump(json_decode('{"key":"value", "":"value"}', true));

Expected result:
----------------
1) array(1) { [""]=> string(5) "value" }
2) array(2) { [""]=> string(5) "value" ["key"]=> string(5) "value" }
3) array(2) { ["key"]=> string(5) "value" [""]=> string(5) "value" }

Actual result:
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1) Page fails to return - some kind of script crash
2) array(2) { ["_empty_"]=> string(5) "value" ["key"]=> string(5) 
"value" } (the "_empty_" key coming up is the most common case when 
there the key is an empty string)
3) array(2) { ["key"]=> string(5) "value" [""]=> string(5) "value" } 
(works as expected)