validate if checkbox has been checked - php

Question

I'm trying to show an error message if a checkbox has not been selected. I've managed to get it done input field, but unsure of how to get it done with a checkbox. This is what I have so far:
<?php
# check if data has been posted
if (!empty($_POST)) {
# keep track validation errors
$author_error = null;
$categories_error = null;
# keep track of post values
$author = $_POST['author'];
$categories = $_POST['categories'];
# validate input
if (empty($author)) {
$author_error = 'Please select author';
$valid = false;
}
if (empty($categories)) {
$categories = 'Please select categories';
$valid = false;
}
# if data is valid, insert into the database
if ($valid) {
}
}
?>
<div class="control-group <?php if (!empty($author_error)){ echo 'error'; } ?>">
<label class="control-label">Author</label>
<div class="controls">
<select name="author" id="author">
<option value="">Select one</option>
<?php $sql2 = 'SELECT id, name FROM author';
foreach ($dbConnection->query($sql2) as $data2) { ?>
<option value="<?php echo $data2['id']; ?>"
<?php if(isset($author) && $author == $data2['id']) { echo 'selected'; } ?>>
<?php echo $data2['name']; ?>
</option>
<?php } ?>
</select>
<?php if (!empty($author_error)) { echo '<span class="help-inline">' . $author_error . '</span>'; } ?>
</div>
</div>
<div class="control-group <?php if (!empty($categories_error)){ echo 'error'; } ?>">
<fieldset>
<legend class="control-label">Categories:</legend>
<?php $sql3 = 'SELECT id, name FROM category';
foreach ($dbConnection->query($sql3) as $data3) { ?>
<div class="controls">
<label for="category<?php echo($data3['id']);?>">
<input type="checkbox" name="categories" id="categories" value="<?php echo($data3['id']); ?>"
<?php if(isset($_POST['categories']) && in_array($data3['id'], $_POST['categories'])) { echo 'checked'; } ?>>
<?php echo($data3['name']); ?>
</label>
</div>
<?php } ?>
</fieldset>
<?php if (!empty($categories_error)) { echo '<span class="help-inline controls">' . $categories_error . '</span>'; } ?>
</div>
Where am I going wrong with the categories field?

Answer 1

Check it like:
isset($_POST['categories'])

Answer 2

On line #62 you are checking if $categories_error is empty but you are using variable $categories for storing error message on line #19