Store and Change Session variable – PHP SESSION VARIABLE

Not sure if anyone understood my problem but if you did and are having something similar I will post how I figured it out.

I simply had to add a session_start(); on my form..

<?php
session_start(); //see here
//this is the form that will change the variable
if(isset($_POST['submit'])) 
{ 
$the_user_garden = $_POST['sub_garden_1'];
}
?>
<form method="post" action="<?php echo $_SERVER['REQUEST_URI']; ?>">
<input type="text" name="sub_garden_1" value="sub_garden_1"><br>
<input type="submit" name="submit" value="Submit Form"><br>
</form>