You must use first setlocale
setlocale(LC_TIME, "fr_FR");
Secondly, for the date of comparison as a string or int, you should still use strtotime, not strftime. (or you can always use a string in the formatYYYY-mm-dd
strftime .
, , . . doc
, ISO 8601 (YYYY-MM-DD) DateTime:: createFromFormat(), .
,
//$next_due_date must be in a standard format like YYYY-mm-dd (but not only)
$due_date = strtotime($next_due_date);
$today_date = time();
if ($due_date > $today_date) {
echo strftime("%c",$due_date) . " is in the future";
}