you can check that by either isset() or empty() (its check explicit isset) weather check box is checked or not
for example
<input type='checkbox' name='Mary' value='2' id='checkbox' />
here you can check by
if (isset($_POST['Mary'])) {
echo "checked!";
}
or
if (!empty($_POST['Mary'])) {
echo "checked!";
}
the above will check only one if you want to do for many than you can make an array instead writing separate for all checkbox try like
<input type="checkbox" name="formDoor[]" value="A" />Acorn Building<br />
<input type="checkbox" name="formDoor[]" value="B" />Brown Hall<br />
<input type="checkbox" name="formDoor[]" value="C" />Carnegie Complex<br />
php
$aDoor = $_POST['formDoor'];
if(empty($aDoor))
{
echo("You didn't select any buildings.");
}
else
{
$N = count($aDoor);
echo("You selected $N door(s): ");
for($i=0; $i < $N; $i++)
{
echo htmlspecialchars($aDoor[$i] ). " ";
}
}