LoginIn check using Session PHP -

i applying check on login page whenever user clicks on login page previous location gets saved session ,

    $_session['httpreferer']=$_server['http_referer']; 

now if user clicks on login page more 1 time have store location first time bacause after logging in redirecting user page came using location have saved in $_session['httpreferer']. problem have tried empty() , have tried isset() have tried null() on $_session['htttpreferr'] nothing working .it keeps on storing location again , again in session .the check not working.

    if( empty($_session['httpreferer']))     {     $_session['httpreferer']=$_server['http_referer'];     } 

please little help.thanks !