PHP, check if the checkbox is checked

Suppose we have a check box array demo[]
<input type="checkbox" name="demo[]" value="1" />A
<input type="checkbox" name="demo[]" value="2" />B
<input type="checkbox" name="demo[]" value="3" />C

To check if any checkbox is checked

   if(!isset($_POST['demo'])){
    echo "You need to check the checkboxs";
   }

 To access the checkbox values using:

$_POST['demo'][0],  $_POST['demo'][1], $_POST['demo'][2]

To make a default checked, for example in the second checkbox

 <input type="checkbox" name="demo[]" value="2" checked />

Result for demo:

A B C