I would like to develop a multilanguage page in PHP, for exemple english/german/japanese.. so when i click on german the page language will change to german, then i click en
You could use sessions to store what language the user has set.
$_SESSION["lang"] = "en";
$_SESSION["lang"] = $_GET["lang"];
// or this way, then you can do example.com/?lang=en
Then create a file with an array of the translation.
$lang["en"]["welcome"] = "Welcome to the site";
And then another file could have the German translation
$lang["de"]["welcome"] = "Willkommen auf der Webseite";
And then you would just call the $lang array with the session
echo $lang[$_SESSION["lang"]]["welcome"];
This is a simplified version of this, you could also use objects to store the translation. But what i do not recommend is to use a database to store the translation, since this involves either a very large SELECT or very very many.