How to keep checkboxes checked after form submit?

I have some checkboxes in an array generated dynamically using a while loop like in the code below. When the checkbox(es) are checked and the form submitted, the value is passed to a PHP variable. However, after the form is submitted and the page loads, the checkboxes do not remain checked. How do I make it so that they stay checked after the form is submitted and the page reloads?

...
<form method="POST">
    <input type="checkbox" name="checkArr[]" value="2">
    <input type="checkbox" name="checkArr[]" value="6">
    <input type="checkbox" name="checkArr[]" value="1">
    <input type="checkbox" name="checkArr[]" value="11">
    <input type="submit" name="submit-form">
</form>
...
<?php
if (isset($_REQUEST['submit-form']) && isset($_REQUEST['checkArr'])) {
    $checkedNumbers = implode(',', $_REQUEST['checkArr']);
}
?>

Thanks in advance.