通过变量从foreach循环发送多个值

I have a foreach loop (see below):

<form action="code/update-to-dispatched.php" method="post" name="markAsDispatched">
    <?php
        foreach ($orders as $row) {
            $_POST['Username'] = $row['Username'];

            echo "<tr class='even'>";
            echo "<td>";
            echo "<strong>Order Date:</strong> ". $row['OrderDate'] ." <br />";
            echo "</td>";           
            echo "<td>";
            echo "<strong>Order ID:</strong> ". $row['OrderID'] ."";
            echo "</td>";
            echo "<td>";
            echo "<strong>Username:</strong> <input type='text' name='username' value=". $row['Username'] ." readonly style='border: 0; background: none;'>";
            echo "</td>";
            echo "<td>";
            echo '<input type="checkbox" name="chkBox[]" id="chkBox" value="'. $row['OrderID'] .'">';
            echo "</td>";
            echo "</tr>";
        }
    ?>

    <span class="tag"><a href="javascript:checkall('markAsDispatched','chkBox',true)">CHECK ALL</a></span>
    <span class="tag"><a href="javascript:checkall('markAsDispatched','chkBox',false)">UNCHECK ALL</a></span>
    <input type="submit" name="markAsDispatched" value="MARK AS DISPATCHED" />
</form>

I currently have 3 orders in my Database so the code above shows 3 orders. I am trying to pass all of the email addresses from the field via the $_POST['Username']. Why does the post variable only equal to the last email rather than a comma separated list such "email1@email.com, email2@email.com, email3@email.com"?

If you want a comma-delimited list, try this:

$usernames = array();
foreach ($orders as $row) {
            //$_POST['Username'] = $row['Username'];
            $usernames[] = $row['Username'];

            echo "<tr class='even'>";
            echo "<td>";
            echo "<strong>Order Date:</strong> ". $row['OrderDate'] ." <br />";
            echo "</td>";           
            echo "<td>";
            echo "<strong>Order ID:</strong> ". $row['OrderID'] ."";
            echo "</td>";
            echo "<td>";
            echo "<strong>Username:</strong> <input type='text' name='username' value=". $row['Username'] ." readonly style='border: 0; background: none;'>";
            echo "</td>";
            echo "<td>";
            echo '<input type="checkbox" name="chkBox[]" id="chkBox" value="'. $row['OrderID'] .'">';
            echo "</td>";
            echo "</tr>";
        }

echo implode(',', $usernames);

You should never override the $_POST parameters by the way, it's very bad practise.