session_start(): Cannot start session when headers already sent

<?php	


    session_start();
    $lang_template = empty($_SESSION['lang_template']) ? 'pc_cn':$_SESSION['lang_template'];
    // dump($lang_template);
    if(isMobiles()){
        if($lang_template == 'pc_cn'){
            return array (
              'cmf_default_theme' => 'pc_cn',
            );
        }elseif($lang_template == 'pc_en'){
            return array (
              'cmf_default_theme' => 'pc_en',
            );
        }else{
            return array (
              'cmf_default_theme' => 'pc_jp',
            );