note 47740 added to function.is-null

I am having problems with a function, it allows me to see whether or not somebody has entered text within the text box. <?php function validatesearch(){ if (is_null($txtEventSearch)) { echo ("events.php?error=true"); } else { echo ("search.php"); } } ?> could anyone tell me what i have done wrong the only part of the function it runs is the echo("events.php?error=true"); ---- Manual Page -- http://www.php.net/manual/en/function.is-null.php Edit -- http://master.php.net/manage/user-notes.php?action=edit+47740 Delete -- http://master.php.net/manage/user-notes.php?action=delete+47740&report=yes Reason: bad code -- http://master.php.net/manage/user-notes.php?action=delete+47740&report=yes&reason=bad+code Reason: spam -- http://master.php.net/manage/user-notes.php?action=delete+47740&report=yes&reason=spam Reason: useless example -- http://master.php.net/manage/user-notes.php?action=delete+47740&report=yes&reason=useless+example Reason: contains commercial links -- http://master.php.net/manage/user-notes.php?action=delete+47740&report=yes&reason=contains+commercial+links Reason: useless note -- http://master.php.net/manage/user-notes.php?action=delete+47740&report=yes&reason=useless+note Reject -- http://master.php.net/manage/user-notes.php?action=reject+47740&report=yes Search -- http://master.php.net/manage/user-notes.php