How do I set the selected item in a drop down box

You can use this method if you use a MySQL database:

include('sql_connect.php');
$result = mysql_query("SELECT * FROM users WHERE `id`!='".$user_id."'");
while ($row = mysql_fetch_array($result))
{
    if ($_GET['to'] == $row['id'])
    {
        $selected = 'selected="selected"';
    }
    else
    {
    $selected = '';
    }
    echo('<option value="'.$row['id'].' '.$selected.'">'.$row['username'].' ('.$row['fname'].' '.substr($row['lname'],0,1).'.)</option>');
}
mysql_close($con);

It will compare if the user in $_GET['to'] is the same as $row['id'] in table, if yes, the $selected will be created. This was for a private messaging system...

You mark the selected item on the <option> tag, not the <select> tag.

So your code should read something like this:

<select>
    <option value="January"<?php if ($row[month] == 'January') echo ' selected="selected"'; ?>>January</option>
    <option value="February"<?php if ($row[month] == 'February') echo ' selected="selected"'; ?>>February</option>
    ...
    ...
    <option value="December"<?php if ($row[month] == 'December') echo ' selected="selected"'; ?>>December</option>
</select>

You can make this less repetitive by putting all the month names in an array and using a basic foreach over them.

Simple and easy to understand example by using ternary operators to set selected value in php

<?php $plan = array('1' => 'Green','2'=>'Red' ); ?>
<select class="form-control" title="Choose Plan">
<?php foreach ($plan as $id=> $value) { ?>
  <option value="<?php echo $id;?>" <?php echo ($id==  '2') ? ' selected="selected"' : '';?>><?php echo $value;?></option>
<?php } ?>
</select>

You need to set the selected attribute of the correct option tag:

<option value="January" selected="selected">January</option>

Your PHP would look something like this:

<option value="January"<?=$row['month'] == 'January' ? ' selected="selected"' : '';?>>January</option>

I usually find it neater to create an array of values and loop through that to create a dropdown.