Description:
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The function json_encode does not encode strings accordingly to the json definition. It affects at least php since 5.3.28 up to the latest version (currently 5.5.11).

According to the definition a string may not contain a quote, slash or control character.
Control characters are the c0 set (0x00-0x1f), delete (0x7f) and the c1 set (0x80-0x9f) (see http://en.wikipedia.org/wiki/Unicode_control_characters).

Source: ext/json/json.c, function json_escape_string

The function only checks for the c0 set but does not handle delete and the c1 set correctly.

The c1 set bug is only affected with the option JSON_UNESCAPED_UNICODE (since php 5.4.0).


Test script:
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echo json_encode(chr(0x7f)).PHP_EOL;

echo json_encode(chr(0xc2).chr(0x80)), JSON_UNESCAPED_UNICODE).PHP_EOL; // the utf8 representation of \u0080


Expected result:
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"\u007f"

"\u0080"


Actual result:
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'"'.chr(0x7f).'"'

'"'.chr(0xc2).chr(0x80).'"' // the utf8 representation of /u0080