php error encountered using stored proc

When calling multiple stored procedures, you can run into the following error: "Commands out of sync; you can't run this command now".
This can happen even when using the close() function on the result object between calls.
To fix the problem, remember to call the next_result() function on the mysqli object after each stored procedure call. See example below:

<?php
// New Connection
$db = new mysqli('localhost','user','pass','database');

// Check for errors
if(mysqli_connect_errno()){
 echo mysqli_connect_error();
}

// 1st Query
$result = $db->query("call getUsers()");
if($result){
     // Cycle through results
    while ($row = $result->fetch_object()){
        $user_arr[] = $row;
    }
    // Free result set
    $result->close();
    $db->next_result();
}

// 2nd Query
$result = $db->query("call getGroups()");
if($result){
     // Cycle through results
    while ($row = $result->fetch_object()){
        $group_arr[] = $row;
    }
     // Free result set
     $result->close();
     $db->next_result();
}
else echo($db->error);

// Close connection
$db->close();
?>