Sessions in word press [duplicate]

Note, Session must start before header is sent. So you should try –

<?php
if( !session_id() ){
    if( headers_sent() ){
        die('headers already sent, cant start session');
    }
    else{
        session_start();
    }
}

// check existence, or not below 1
if( !isset($_SESSION['impression']) || $_SESSION['impression'] < 1 ){
    $_SESSION['impression'] = 100;  
}

// decrease the value by one
$_SESSION['impression']--;

// test
echo $_SESSION['impression'];
?>