How to access a $_SESSION variable inside of modX

<?php
$sessionId = session_id();
if(!isset($sessionId)) {
    session_start();
}
define('SESSION_ID',session_id());
echo SESSION_ID;
?>

<form class="form-inline home1"  id="home1" role="form" action="[[++site_url]][[~17]]" method="POST">

            <div id="input-wrap" class="">
            
              <div class="form-group ">
                <label class="sr-only" for="">Zip Code</label>
                <input type="text" class="form-control" placeholder="Zip Code" name="zip" id="zip" style="max-width: 120px;">
              </div>
                
              <div class="form-group pull-right" >
                <label class="col-lg-3 sr-only control-label">Products</label> 
                <select class="form-control" name="product" style="width:140px;">
                  <option value="" selected>Product</option>
                  <option value="Auto">Auto</option>
                  <option value="Boat">Boat</option>
                  <option value="Commercial">Commercial</option>
                  <option value="Commercial Property">Commercial Property</option>
                  <option value="Condo">Condo</option>
                  <option value="Homeowners">Homeowners</option>
                  <option value="Life">Life</option>
                  <option value="Motorcycle">Motorcycle</option>
                  <option value="Renters">Renters</option>
                  <option value="Workers Compensation">Workers Compensation</option>
                </select>
              </div>
            
            </div><!-- close input wrap -->
                          
            <button id="next1" class="btn btn-orange btn-block" type="submit" >Get Started</button>
           
          </form>

[[11]]

<?php

$var = "Hello World";
return $var;

// if(isset($_POST['zip'])){
// 	$zip = mr_clean($_POST['zip']);
// 	return $zip;

// }
// else{
//     echo "no post";
// }