This function is, surprisingly, properly documented. I was working on resolving 
this bug, when I checked the sources for the function, and found that there is 
no non-error situation where the function will return anything but an encoded 
string.

The reason for this is all types can be encoded, including null. There are no 
values which you could pass to json_encode that would not result in a string 
being returned. The only way for it to fail is for you to not pass it an 
incorrect number of parameters, or some other error in syntax.

Not all types can encode. For example: Resource.

php -r "var_dump(json_encode(fopen('/tmp/temp', 'w+')));"
PHP Warning:  json_encode(): type is unsupported, encoded as null in Command line 
code on line 1
string(4) "null"

But, I agree that it anyway returns a string ("null" string)

That being the case, would it be worth modifying the "Return Values" section in the documentation from "Returns a JSON encoded string on success.", since there seems to be no failure case as "on success" seems to imply? something like "Returns a JSON encoded string in all cases" or possibly something mentioning the null return case?