Zend_Session: Session must be started before any output has been sent to the browser

Question

I've run into this issue before, but I can't remember how to solve it. I have created a bare bones (can't get any simpler) controller, and am just trying to echo something to the browser, and I am getting this message:

Fatal error: Uncaught exception 'Zend_Session_Exception' with message 'Session must be started before any output has been sent to the browser ...

Here's my entire controller. It's displaying 'success', but it also displays the error message. How can I silence that error message so I can simply echo something to the browser?

<?php

class CacheController extends Zend_Controller_Action
{
    public function clearAction()
    {
        $this->_helper->layout->disableLayout();
        $this->_helper->viewRenderer->setNoRender();
        try {
            $result = Model_Cache::emptyCache(array('foobar'=>1));

            if ($result['status'] == true) {
                echo 'Success';
            } else {
                echo 'Error: ' . $result['message'];
            }
        } catch (Exception $e) {
            echo 'Error: ' . $e->getMessage();
        }
    }
}

Answer 1

From memory, this error is usually due to non-PHP code starting the output before intended (and session initialisation). Usually it's due to whitespace or carriage returns after unnecessary ?> tags. This is the first thing I'd check for.