The t3lib_div:devlog() function is defined as follows:

/**

 * Developer log; This should be implemented around the source code, both frontend and backend, logging everything from the flow through an application, messages, results from comparisons to fatal errors.

 * The result is meant to make sense to developers during development or debugging of a site.

 * The idea is that this function is only a wrapper for external extensions which can set a hook which will be allowed to handle the logging of the information to any format they might wish and with any kind of filter they would like.

 * If you want to implement the devLog in your applications, simply add lines like:

 *         if (TYPO3_DLOG)    t3lib_div::devLog('[write message in english here]', 'extension key');

 *

 * @param    string        Message (in english).

 * @param    string        Extension key (from which extension you are calling the log)

 * @param    integer       Severity: 0 is info, 1 is notice, 2 is warning, 3 is fatal error, -1 is "OK" message

 * @param    array         Additional data you want to pass to the logger.

 * @return   void

 */

function devLog($msg, $extKey, $severity=0, $dataVar=FALSE)     {

I suggest to use the  t3lib_div:devlog() function like this:

class t3lib_userAuth {

    // write messages into the devlog?

var $writeDevLog = FALSE;

function start() {

    global $TYPO3_CONF_VARS;

        // enable dev logging if set

    if ($TYPO3_CONF_VARS['SC_OPTIONS']['t3lib/class.t3lib_userauth.php']['writeDevLog'])

        $this->writeDevLog = TRUE;

    if (TYPO3_DLOG) $this->writeDevLog = TRUE;

    ...

    if ($this->writeDevLog AND !is_array($this->user))        t3lib_div::devLog('No user session found.', 't3lib_userAuth', 2);