Description:
------------
Invalid UTF-8 string throws E_WARNING depending on the configuration directive display_errors:

ini_set('display_errors', 0);
json_encode("\xFF");  // PHP Warning:  json_encode(): Invalid UTF-8 sequence in argument (in error log)

ini_set('display_errors', 1);
json_encode("\xFF");  // *nothing*

This directive determines whether errors should be printed to the screen as part of the output or if they should be hidden from the user. It should not be used to change json_encode behaviour.

The same bug affects htmlentities() and htmlspecialchars() 

Test script:
---------------
ini_set('display_errors', 0);

json_encode("\xFF");

htmlspecialchars("\xFF", ENT_COMPAT, 'utf-8');

htmlentities("\xFF", ENT_COMPAT, 'utf-8');