How to insert multiple selects into separate rows

Having read several posts on this subject, I have managed to insert data from a multiple select form.
However, the data goes into one row. I need each value to go into separate rows.
Here is the relevant field in the form

<p>
    <label>Assign Users::</label>
    <select id="client" name="client[]"  multiple="multiple">
        <option  class="multiple" value="selected">Assign User to Project</option>
        <?php 
            foreach($clients as $client){ 
        ?>
        <option value="<?php echo $client ->user_id;?>"><?php echo $client ->user_name; ?></option>
        <?php 
        }
        ?>
    </select>
</p>

And here is the Insert code

$ref=$_POST['project_ref'] ;
$user_id=implode(',', $_POST['user']);
$wpdb->insert( 'vo_wp_assigned_projects', array('client'=>$user_id,'project'=> $ref));

This inserts into one row, which is no use as I need to be able to access each individual entry.
I have tried foreach loops from other posts in this group with no success.
I hope I have provided sufficient information.