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<?php
if(isset($_SESSION['myKey'])) {
    $value = $_SESSION['myKey'];
} else {
    $value = '';
}
?>

<?php
add_action('init', 'myStartSession', 1);
//add_action('wp_logout', 'myEndSession'); //Uncomment this if you need destroy session on logout
//add_action('wp_login', 'myEndSession'); //Uncomment this if you need destroy session on logout

function myStartSession() {
    if(!session_id()) {
        session_start();
    }
}
//Uncomment line for activate destroy session
//function myEndSession() {
//    session_destroy ();
//}
?>

<?php
$_SESSION['myKey'] = "Some data I need later";
?>