// Prepare PKCE code_challenge
        $code_verifier = substr( str_shuffle("abcdefghijklmnopqrstuvwxyz0123456789ABCDEFabcdefghijklmnopqrstuvwxyz0123456789ABCDEFabcdefghijklmnopqrstuvwxyz0123456789ABCDEF"), 0, 50);
        $code_challenge = strtr(rtrim(base64_encode(hash('sha256', $code_verifier, true)), '='), '+/', '-_');
        $_SESSION['code_verifier'] = encrypt($code_verifier);  
...