json_encode returns null for certain strings. See: http://bugs.php.net/49588

json_encode bug

<?php
$a = "Hello, I'm an other charset éc éc";
$b = htmlentities($a, ENT_QUOTES, 'UTF-8');
$c = htmlentities($a);

echo "\n", json_encode($a); //ouput will be "Hello, I'm an other charset "
echo "\n", json_encode($b); //ouput will be "" (NULL)
echo "\n", json_encode($c); //ouput will be "Hello, I'm an other charset &eacute;c &eacute;c"

If the file is encoded correctly

string(45) ""Hello, I'm an other charset \u00e9c \u00e9c""
string(54) ""Hello, I'm an other charset éc éc""
string(61) ""Hello, I'm an other charset éc éc""

Thanks for your comment. I thought I still can check the character encoding based on input (signed - but still under ACSII encoding) but forgot about the file encoding (when I test). And the "character set" of the DB too :(