input-checkbox-2.php

<?php
// an array of values
$vitamins = array( 'Vitamin A', 'Thiamin (B1)', 'Riboflavin (B2)', 'Niacin (B3)', 'Pantothenic Acid (B5)', 'Vitamin B6', 'Vitamin B12', 'Vitamin C', 'Vitamin D', 'Vitamin E', 'Vitamin K' );

// stores _food_vitamins array 
$current_vitamins = ( get_post_meta( $post->ID, '_food_vitamins', true ) ) ? get_post_meta( $post->ID, '_food_vitamins', true ) : array();
?>
<div class='inside'>
	<h3><?php _e( 'Vitamins', 'food_example_plugin' ); ?></h3>
	<p>
	<?php
	foreach ( $vitamins as $vitamin ) {
	?>
		<input type="checkbox" name="vitamins[]" value="<?php echo $vitamin; ?>" <?php checked( ( in_array( $vitamin, $current_vitamins ) ) ? $vitamin : '', $vitamin ); ?> /><?php echo $vitamin; ?> <br />
	<?php
	}
	?>
	</p>
</div>