How do I make check appear in an edit form?

Basically I have a form that I start out with. On that form is the option to select a checkbox and once I click submit on that form, the information is read into a database and the condition on whether the checkbox has been selected now displays a '0' or '1' in the database.

Now I have an edit form. I choose the row I want to edit and my edit form gets populated with what is in the row. The only thing that doesn't show up, is whether that row was 'checked' on the first form. I want the checkbox to be marked if the user checked the checkbox on the first form and visversa.

Sorry if this sounds like a terrible explination.

Code for the first form:

<form method="post" action="processForm.php">

    Name: <input type="text" name="names" required = "required"><br>

    Activate: <input type="checkbox" name="activateBox"><br>

    <input type="submit" value="Create Users"><br>
</form>

And code for edit form:

<form method="post" action="editForm.php">

    ID: <input type="text" readonly name="id" value="<?php echo $row['id']; ?>"><br><br>
    First Name: <input type="text" name="firstName" value="<?php echo $row['firstName']; ?>"><br><br>
    Last Name: <input type="text" name="lastName" value="<?php echo $row['lastName']; ?>"><br><br>
    Email: <input type="text" name="email" value="<?php echo $row['email']; ?>"><br><br>
    Activate: <input type="checkbox" name="activateBox" value="<?php echo $row['activated']; ?>"><br><br>

    <input type="submit" value="Update"><br>

</form>

I tried doing this but it did nothing. I'm sure I'm missing something pretty simple but I can't figure out what.

if (isset($_POST['activateBox'])) {
    echo 'checked="checked"';
}