Simple functions to set and retrieve alert messages in a browser session in WordPress. (Find and replace `_s_` with your own namespacing.)

wordpress-alert-messages.php

<?php

// Start browser session (to store alert messages)
function _s_start_browser_session() {
    if(!isset($_SESSION)) {
        session_start();
    }
}
add_action('init', '_s_start_browser_session');


// Store alert message in browser session
function _s_set_alert_message( $category, $code, $message ) {
    $_SESSION[$category][$code] = $message;
}


// Get alert message from browser session
function _s_get_alert_message( $category, $code ) {
    $message = '';
    if ( isset($_SESSION[$category][$code]) && $_SESSION[$category][$code] != '' ) {
        $message = $_SESSION[$category][$code];
        unset($_SESSION[$category]);
    }
    return $message;
}

?>