The modern and rock-solid approach is the intl (from "Internationalization") extension, which offers e.g. the IntlDateFormatter
class:
$date = new DateTime('2016-06-14');
$fmt = new IntlDateFormatter('fr_FR', IntlDateFormatter::LONG, IntlDateFormatter::NONE, 'Europe/Paris', IntlDateFormatter::GREGORIAN);
var_dump($fmt->format($date));
string(12) "14 juin 2016"
If you think it's overkill for your project, you can use the legacy strftime() function but you need to change current locale:
$date = strtotime('2016-06-14');
var_dump(setlocale(LC_TIME, 'fr_FR', 'fr')); // Need to try values until you get true
var_dump(strftime('%B %e %Y', $date));
You need to have French locale data installed. In my experience, this works better on Unix-like systems than on Windows.