PHP Session SID URL - url does not have SID - php

Question

I'm trying to learn how to use Sessions to maintain state between pages.
I think my problem is that I dont know how to escape properly. When I load the page im not getting the SID at the end of the url, instead I'm getting this:
mytestsite.com/login.php?<?php echo SID;?>
I have session_start() at the top of every page. And the session initialy works as I see the login name appear on the page and the Register and Login links, but then when I click to another page the session is not maintained.
The main site nav is dynamically createdusing an array($nav) and foreach loop. Here too I'm not sure how to echo the url and php SID constant. I don't think my syntax is correct.
//LOGIN BAR
echo "<nav id='statusBar'>";
echo "<ul>";
if(isset($_SESSION['userIn']) && $_SESSION['userIn'] != ''){
echo "<li id='userLo'>Log Out</li>";
echo "<li id='userLi'>User Logged In: " . $_SESSION['userIn'] . "</li>";
} else{
echo '<li>Register</li>';
echo '<li>Login</li>';
}
echo "</ul>";
echo "</nav>";
//START MAIN MENU
echo "<nav id='nav'> ";
$nav = array();
$nav['index.php?<?php echo SID;?>'] = 'Home';
$nav['public1.php?<?php echo SID;?>'] = 'Public 1';
$nav['public2.php?<?php echo SID;?>'] = 'Public 2';
$nav['members.php?<?php echo SID;?>'] = 'Members';
//CREATE UNORDERED LIST
echo '<ul>';
foreach ($nav as $key => $navValues) {
echo "<li>";
echo "<a href='$key'>$navValues</a>";
echo "</li>";
}
echo '</ul>';
echo "</nav>";

Answer 1

Below everything within echo '..'; is interpreted as html.
echo '<li>Login</li>';
If you want to use variables inside the string you have to concatenate them like so:
echo 'Login' ;
For concatenating php code take a look at:
http://php.net/manual/en/language.operators.string.php
and more about sessions:
http://php.net/manual/en/session.constants.php