Question

Is it considered normal using session_start on a class constructor when session values need to be retrieved or keep? Best practices?

Session variable coudn’t be retrieved from class unless session_start is called from __contruct.

session_start();    

if(isset($_REQUEST['siteid'])){
    $siteid = $_REQUEST['siteid'];
    $_SESSION['siteid'] = $siteid;
    echo $siteid;
}else{
    $siteid = "";
}
require_once 'common/lib_constant.php';
require_once 'common/database.php';
require_once 'common/common.class.php';
$commonClass = new commonClass();

Answer 1

best practice is to start session from the bootstrap.
and session itself accessing through some kind of a smart wrapper class (see ZF for examples).
That way you should be 100% sure you won’t have header problems and duplicate session_start calls in your code.
A class can be instantiated many times/request…bootstrap runs only once/request.