jQuery is not defined, working on local server but not online

Add this in functions.php instead of header.php

function frontEnd_scripts() {
    wp_enqueue_script("jquery");
    wp_deregister_script('jquery.mobile');
    wp_register_script('jquery.mobile', ("//ajax.googleapis.com/ajax/libs/jquerymobile/1.4.3/jquery.mobile.min.js"),array('jquery'));
    wp_enqueue_script('jquery.mobile');
    wp_register_style('jqm_css',('//ajax.googleapis.com/ajax/libs/jquerymobile/1.4.3/jquery.mobile.min.css'));
    wp_enqueue_style('jqm_css');
}
add_action('wp_enqueue_scripts', 'frontEnd_scripts');