Echoing checkbox values to use for DELETE query is failing

I cannot seem to understand why this is not echoing:

foreach($_POST["checked"] as $value) {
    echo "$value";
}

When I just use the following:

echo $_POST['checked'];

The value is shown but only for 1 checkbox.

I need to grab all the values of all checked checkboxes.

This is my checkbox:

echo '<td><input id="checked" name="checkbox[]" type="checkbox" value="'.$row['id'].'"></td>';