PHP UPDATE with if and else statement - php

Question

I need a little help with my script, I am making a library sign in/out and page and I have it working so that it posts the form when they sign in and out, but i want to make it to where if they are trying to sign out and they actually never signed in it would pop-up a js alert tell them that and I can't quite get it.
Here's the code:
<?php
session_start();
include_once("connect.php");
date_default_timezone_set("America/Winnipeg");
$date = ("m-d-Y");
$timeout = date("g:i:s a");
//search for existing entries
if ("SELECT EXISTS(SELECT * FROM signin_out WHERE
lname='".$_POST['lastname']."' AND fname='".$_POST['firstname']."'
AND date='".$date."')") {
//if they exist run this
mysql_query("UPDATE signin_out SET timeout='" . $timeout . "'
WHERE lname='" . $_POST['lastname'] . "'
AND fname='" . $_POST['firstname'] . "' AND timeout='' ");
header("Location: ../index.html");
//if they don't exist run this
} else {
header("Location: ../index.html");
echo "<script type='text/javascipt'>\n";
echo "alert('You did not sign in!');\n";
echo "</script>";
}
?>

Answer 1

The HTTP Location header already sends them to ../index.html long before the JavaScript gets echoed out. You need to have a logout.php page with that JavaScript or a $_GET variable for the page you redirect them to, and trigger the alert box there.
For instance:
...
} else {
header('Location: ../index.php?notloggedin');
}
And somewhere in your index.php:
<?php
if(isset($_GET['notloggedin'])) {
echo '<script>alert("You did not sign in!");</script>';
}
?>
Although I find that an alert box is rather intrusive, I'd rather do:
<?php
if(isset($_GET['notloggedin'])) {
echo '<p><strong>You did not sign in!</strong></p>';
}
?>