Keeping a Checkbox checked in a checkbox group if error on submit PHP

I have a group of checkboxes and I need to keep them checked when a submit error occours. It validates and will display an error if no selection is made, but won't keep the boxes selected if it postsback.

HTML

<div class="fieldBox">
<div class="label"><p class="bold">Special Needs</p></div>
<div class="field"><?php echo $snMessage; ?>
<input type="checkbox" id="none" value="None" name="sNeeds[None]" <?php   echo(in_array('None',$sNeeds))?'checked="checked"':'';?> /><label for="none">   <span></span>None</label>
<input type="checkbox" id="IEP" value="IEP" name="sNeeds[IEP]" <?php echo(in_array('IEP',$sNeeds))?'checked="checked"':'';?> /><label for="IEP"><span></span>IEP</label>
<input type="checkbox" id="405" value="405" name="sNeeds[405]" <?php echo(in_array('405',$sNeeds))?'checked="checked"':'';?> /><label for="405"><span></span>405</label>
</div>
</div> 

PHP

$sNeeds = array('None', 'IEP', '405');
//Check Special Needs
if(empty($_POST['sNeeds'])) {
    $snMessage .= '<p class="errorClass">Required</p>';
}

Where did I go wrong? Thank you in advance for your time!