<?php
//this is default variable if nothing is submitted with my form
$the_user_garden = "gardens";
?>

<?php
//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>

<?php
//this put the new variable value   from the form as session variable
$_SESSION['myKey'] = $the_user_garden;
?>

<?php
$author_id = get_current_user_id( );
//this is where I am using the changes variable
if( have_rows($the_user_garden, 'user_'.$author_id) ): ?>

<?php
//and here too
while( have_rows($the_user_garden, 'user_'.$author_id) ): the_row(); ?>

<?php
add_action('init', 'myStartSession', 1);
add_action('wp_logout', 'myEndSession');
add_action('wp_login', 'myEndSession');

function myStartSession() {
if(!session_id()) {
    session_start();

}
}

function myEndSession() {
session_destroy ();
}

?>
<?php
// this is where I need the variable for gravity form(pop up)
 if(isset($_SESSION['myKey'])) {
$the_user_garden = $_SESSION['myKey'];
} else {
$the_user_garden = "gardens";

}
?>

<?php
//this is where I am using variable
$author_id = get_current_user_id( );
if( have_rows($the_user_garden, 'user_'.$author_id) ): ?>