json_encode() behavior and the browser

I'm curious if the behavior of json_encode() is influenced by the browser at all. I have a page that returns search results. If I access the page and perform a search using Chrome, the following error shows up in the log: PHP Warning: json_encode() [<a href='function.json-encode'>function.json-encode</a>]: Invalid UTF-8 sequence in argument in [PAGE] on line [LINE] If I access the page and perform a search, the exact same search using the exact same parameters, using Firefox then I get the expected results. When I var_dump() the return value of json_encode(), I see that it is a null in the case where I accessed using chrome but the expected string in the case where I accessed using firefox. In both cases, the input array is identical. Given the identical input and different output, the only thing I can figure is that the headers sent to the server as part of the request figure in to how json_encode() behaves. Is that the case? Or am I barking up the wrong tree? To be clear, I'm not talking about how the browser ultimately handles the json encoded data. I know there can be issues with that. I'm talking about the process before the data is even shipped to the browser -- about how json_encode() behaves when executed as part of the PHP script. thnx, Christoph