php form process multiple checkboxes

<?php
/* Scrub form input using check_input function */
$firstName = check_input($_POST['firstName']);
$lastName = check_input($_POST['lastName']);
$address  = check_input($_POST['address']);
$city = check_input($_POST['city']);
$zip = check_input($_POST['zip']);
$crossStreets = check_input($_POST['crossStreets']);
$phone = check_input($_POST['phone']);
$areaCode = check_input($_POST['areaCode']);
$phone1 = check_input($_POST['phone1']);
$phone2 = check_input($_POST['phone2']);
$cell = check_input($_POST['cell']);
$email = check_input($_POST['email']);
$otherDetails = check_input($_POST['otherDetails']);
$apptDate = check_input($_POST['apptDate']);
$apptTime = check_input($_POST['apptTime']);
$comments = check_input($_POST['comments']);

/* Handle checkbox arrays */
$flooringType = implode(', ', $_POST['flooringType']);
$roomsFlooring = implode(', ', $_POST['$roomsFlooring']); 

/* Send the message using mail() function */
mail("...edited on purpose to save forum space" );

/* Redirect visitor to the thank you page */
header('Location: thankyou.php');
exit();

/* Functions to scrub data and error message for required form fields */
function check_input($data, $problem='')
{
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
    if ($problem && strlen($data) == 0)
    {
        show_error($problem);
    }
    return $data;
}

function show_error($myError)
{
?>

<script language="javascript" type="text/javascript">
alert('Please fill in the following information:\n\n<?php echo $myError; ?>');
onclick = history.back(); 
</script>
<noscript>Please fill in the following information:
<?php echo $myError; ?></noscript>

<?php
exit();
}
?>