Php访问类上的静态方法

First all i'm new on OOP. Sorry if this is a noob question.

I'm building a multilanguage web site. I'm fetching database values with languga iso's. For eg: index.php?lang=en if language value is set with $_GET parameter, mysql brings only en values in database. The problem is with my LanguageController.

// LanguageController Class
class LanguageController {
    public $lang = "en";
    public function __construct() {
        return true;    
    }
    public static function detectLang() {
        $lang = 'en';
        ob_start();
        session_start();
        if(isset($_GET["lang"])) {
            $lang= $_GET["lang"];
            $_SESSION["lang"] = $lang;
            setcookie("lang", $lang, time() + (3600 * 24 * 30));
        }
        else {
            $lang = 'en';
            $_SESSION["lang"] = $lang;
            setcookie("lang", $lang, time() + (3600 * 24 * 30));
            return true;
        }
    }
}

I want to pass default $lang parameter for eg 'en'. If client change language to 'fr', i must start a session. And mysql brings 'fr' language values. How can i pass my $lang parameter on my detectLang() method.

I tried var_dump(LanguageController::detectLang()) It's coming NULL.

Any help greatly appricated.

When there's no return statement in a function - returned value is NULL. Obviously your detectLang function works and the isset($_GET["lang"]) condition is true, in this case the returned value is NULL.

In static methods you can only reference to static attributes. In simple words, the $lang needs to be static as well i.e.

public static var $lang = "en";