If your check box markup looks like:
<input type="checkbox" name="myCheckBox" value="Yes" />
The check box "mycheckbox" would have a value in the post array only if the checkbox is checked. Otherwise the value would be nothing. so ensure that you check if the value is set and to checked. You can do it like:
<?php
if(isset($_POST['myCheckBox']) && $_POST['myCheckBox'] == 'Yes')
{
echo "checked.";
}
else
{
echo "not checked";
}
?>