I'm trying to mimic the json_encode bitmask flags implemented in PHP 5.3.0, here is the string I have:

$s = addslashes('O\'Rei"lly'); // O\'Rei\"lly

Doing json_encode($str, JSON_HEX_APOS | JSON_HEX_QUOT) outputs the following:

"O\\\u0027Rei\\\u0022lly"

And I'm currently doing this in PHP versions older than 5.3.0:

str_replace(array('\\"', "\\'"), array('\\u0022', '\\\u0027'), json_encode($s))
or
str_replace(array('\\"', '\\\''), array('\\u0022', '\\\u0027'), json_encode($s))

Which correctly outputs the same result:

"O\\\u0027Rei\\\u0022lly"

I'm having trouble understanding why do I need to replace single quotes ('\\\'' or even "\\'" [surrounding quotes excluded]) with '\\\u0027' and not just '\\u0027'.