jQuery ajax pass variable to php session, but php session not store it

Question

I try to add class to body, and store it in php session. I use ajax to do it, but php session not store it. Variable is passing correct. Session is started in php function.

(function ($) {
    jQuery(document).on('click', '#wcag-button', function () {
        $('body').attr('id', 'wcag-theme');
        $('#wcag-button').hide();
        $('#wcag-button-off').show();

        var data = {
            'action': 'wcag_contrast_on',
            'bodyClass': 'wcag-theme'
        }
        $.post(Theme_Variables.ajax_url, data, function(response) {
            console.log(response);
        });
    });
})(jQuery);


add_action('wp_ajax_wcag_contrast_on', 'wcag_contrast_on');
add_action('wp_ajax_nopriv_wcag_contrast_on', 'wcag_contrast_on');

    function wcag_contrast_on() {
        session_start();
        $body_class=$_POST['bodyClass'];
        $_SESSION["usertemplate_css"] = $_POST['bodyClass'];
        //echo 'You sent ' . $body_class;
        echo ($_SESSION["usertemplate_css"]);
        $response['message']    = "Successfull Request";
        echo json_encode($response);
        wp_die();
    }

I also try to start session in header, but not working.

Answer 1

Please try the below solution Using WordPress Init Hook

add_action('init', 'set_session'); 
function set_session() {
 if(!session_id()){
      session_start();
  }
}