HTML:

<input type="checkbox" name="input_name[]" value="test"/>
<input type="checkbox" name="input_name[]" value="test1"/>
<input type="checkbox" name="input_name[]" value="test2"/>

PHP:
<?php
//you should check if input_name is set
if(sizeof($_POST['input_name']))
	echo implode(', ', $_POST['input_name']);
?>