Headers already sent when I try to add headers via a shortcode

Question

I've run into a headers already send problem while adding functionality to one of my plugins. I wish to send my 301 redirect header after some checks to deteremine if the contents of the page is obsolete. I have a wordpress page that contains just the shortcode [shortcode]. Then in my plugin I hook into init to register my shortcode using the following snippet of code

function __construct()
{
    //register an activation hook for the plugin
    register_activation_hook(__FILE__, array(
        &$this,
        'install_bnw'
    ));
    //Hook up to the init action
    add_action('init', array(
        &$this,
        'start_session'
    ), 1);
    add_action('init', array(
        &$this,
        'class_loader'
    ));
    add_action('init', array(
        &$this,
        'init_bnw'
    ));
    add_filter('wpseo_canonical', array(
        &$this,
        'wpseo_canonical'
    ));
}


/**
 * Runs when the plugin is activated
 */
function install_bnw()
// TODO: Add check if the *** plugin is currently activated and if it can be reached from this plugin.
{

    // do not generate any output here
}

/**
 * Starts a session before any output is generated.
 * TODO: Might not be the most efficient way to handle sessions, needs improvement.
 */
public function start_session()
{
    if (!session_id()) {
        session_start();
    }

}

/**
 * Runs when the plugin is initialized.
 * Adds filters to wordpress functions and calls functions that need to be executed.
 */
function init_bnw()
{     // Adds all the shortcodes that need to be added.
    $this->add_shortcodes();
  // -- Some more code -- 
}

/**
 * Autoloader that uses template_autoloader to load class files from a predetermined location.
 */
function class_loader()
{
    // register an autoloader function for template classes
    spl_autoload_register(array(
        &$this,
        'template_autoloader'
    ));
}


/**
 * Used by the class_loader function to load classes from files but only if the files exist to ensure this plugin works with other plugins.
 * @param $class
 * @return bool that tells if the include worked.
 * @throws Exception
 *
 */
function template_autoloader($class)
{
    $classPath = "includes/$class.php";
    // Checks if the file exists for compatibility with other plugins using the spl_autoload functionality.
    if (file_exists(__DIR__ . '/' . $classPath)) {
        include_once $classPath;
        return true;
    } else {
        return false;
    }

}



/**
     * Contains all the shortcodes that need to be added to wordpress in order for this plugin to function.
     */
    private function add_shortcodes()
    {
       // Add all the shortcodes that need to be added here.
     // --Some shortcodes here --
    add_shortcode('shortcode', array(
            &$this,
            'shortcodeDesinationFunction'
        ));
       // -- More shortcodes --        
    }

The function shortcodeDestinationFunction loads a class, with an empty constructor and then calls the following function:

function showTraining()
{
    // -- Do some data getting and business logic here but no outputting --
    // If training is in the past
    if($startdate < $currentdate)
    {
// -- Some more data getting function that don't output anthing
        $newLocation = "/text/text/$subjectLabel/$trainingLevel";
        // Disable caching for testing
         header('Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0');
         header('Expires: Sat, 26 Jul 1997 05:00:00 GMT'); // past date to encourage expiring immediately
        header ('HTTP/1.1 301 Moved Permanently');
        header ('Location:'.$newLocation); 
    }
 // Some more code

As you can see I want the page that has this shortcode to be able to send a 301 redirect when a certain condition applies.

When I run the code I get the following error's (from the blackbox wordpress plugin):

Warning Cannot modify header information - headers already sent by (output started at /var/www/vhosts/bndev.int.bijlesnetwerk.nl/httpdocs/wp-includes/class.wp-styles.php:122) on line 183 in file /var/www/vhosts/bndev.int.bijlesnetwerk.nl/httpdocs/wp-content/plugins/BNWPweb/includes/examenTraining.php
Warning Cannot modify header information - headers already sent by (output started at /var/www/vhosts/bndev.int.bijlesnetwerk.nl/httpdocs/wp-includes/class.wp-styles.php:122) on line 184 in file /var/www/vhosts/bndev.int.bijlesnetwerk.nl/httpdocs/wp-content/plugins/BNWPweb/includes/examenTraining.php
Warning Cannot modify header information - headers already sent by (output started at /var/www/vhosts/bndev.int.bijlesnetwerk.nl/httpdocs/wp-includes/class.wp-styles.php:122) on line 186 in file /var/www/vhosts/bndev.int.bijlesnetwerk.nl/httpdocs/wp-content/plugins/BNWPweb/includes/examenTraining.php
Warning Cannot modify header information - headers already sent by (output started at /var/www/vhosts/bndev.int.bijlesnetwerk.nl/httpdocs/wp-includes/class.wp-styles.php:122) on line 187 in file /var/www/vhosts/bndev.int.bijlesnetwerk.nl/httpdocs/wp-content/plugins/BNWPweb/includes/examenTraining.php 

Line 183 is the first header statement, and the line referenced from the wordpress core file contains the following:

} else {
            echo $tag;
            $this->print_inline_style( $handle );
        }

Now my question is, how do I send a header via a plugin shortcode without getting the headers already send warning?

I have already googled quite a bit and I have read some stackexchange answer but none seem applicable, I don't think it's a problem with a random whitespace or one of my plugins outputting something that it isn't supposed to. I have also read Wordpress FAQ about headers already sent to no avail.

Answer 1

You can't add headers after content has already been sent to the browser. If you try, you will get those "Warning Cannot modify header information - headers already sent by... " messages.

Shortcodes execute inside the the_content filter, and that filter runs well into a page load. That means that content has long since been sent to the browser by the time you try to add headers. To make this work with a shortcode you'd need to do something like what is done here, but honestly, I'd recommend not using a shortcode. That isn't the right tool for altering early page load content.

I can't tell exactly what you are doing, but my gut feeling is that a custom meta field would be a better choice.