Requiring Fields in Safari 5.1 - php

Question

As stated here, the required attribute in HTML5 doesn't work in Safari. I have PHP that checks if the required fields are filled out. When the user submits the form (a contact-us type form), the action="" is set to the same page, so that the PHP runs. It sends the email with the information and redirects to the home page (which I tell users at the bottom of the page).
However, I heard about people being able to change the HTML of the page and not require fields. While the form is just a contact form and there's no reason for anyone to go through the trouble of modifying the code, I'm relatively new to web development and programming, and I'd like to know how to prevent this kind of stuff if I was to work on projects in the future where I'd need this much security.
So, anyway, the PHP successfully does not send the email if the required information isn't filled out, but if it isn't filled out, it redirects to the home page anyway. How would I use PHP to stay on the same page after the required information isn't there? I know I can use JavaScript, and I probably will in addition to PHP, but since they can have that turned off, it's not totally secure. Again, I know that my situation doesn't require this much security, but it's good practice for a beginner in my opinion. Here's what I have so far:
<?php
if (isset($_POST['email']) && isset($_POST['address']) && isset($_POST['floors'])) {
$name = $_POST['name'];
$lotSize = $_POST['lotSize'];
$lotSize2 = $_POST['lotSize2'];
$age = $_POST['age'];
$taxes = $_POST['taxes'];
$notes = $_POST['notes'];
$email = $_POST['email'];
$address = $_POST['address'];
$floors = $_POST['floors'];
$bedrooms = $_POST['bedrooms'];
$bathroomsFull = $_POST['bathroomsFull'];
$bathroomsHalf = $_POST['bathroomsHalf'];
if (isset($_POST['basement'])) {
$basement = "Yes";
} else {
$basement = "No";
}
if (!(empty($email) && empty($address) && empty($floors) && is_numeric($floors))) {
if ((strlen($name) < 101) && (strlen($email) < 255) && (strlen($address) < 51) && (strlen($lotSize) < 8) && (strlen($floors) < 4) && (strlen($age) < 4) && (strlen($taxes) < 8) && (strlen($notes) < 481)) {
$message = "You have received information about a listing!<hr/><br/>
<table style='border:1px solid black;padding:3px'>
<tr style='padding:5px'><td><strong>Name:</strong></td><td>" . $name . "</td></tr>
<tr style='padding:5px'><td><strong>Email:</strong></td><td>" . $email . "</td></tr>
<tr style='padding:5px'><td><strong>Address:</strong></td><td>" . $address . "</td></tr>
<tr style='padding:5px'><td><strong>Lot size:</strong></td><td>" . $lotSize . " × " . $lotSize2 . " sq. ft. (" . ($lotSize*$lotSize2) . " sq. ft.)</td></tr>
<tr style='padding:5px'><td><strong>Floors:</strong></td><td>" . $floors . "</td></tr>
<tr style='padding:5px'><td><strong>Bedrooms:</strong></td><td>" . $bedrooms . "</td></tr>
<tr style='padding:5px'><td><strong>Full Baths:</strong></td><td>" . $bathroomsFull . "</td></tr>
<tr style='padding:5px'><td><strong>Half Baths:</strong></td><td>" . $bathroomsHalf . "</td></tr>
<tr style='padding:5px'><td><strong>House Age:</strong></td><td>" . $age . " years</td></tr>
<tr style='padding:5px'><td><strong>Taxes:</strong></td><td>$" . $taxes . " / Yr.</td></tr>
<tr style='padding:5px'><td><strong>Basement:</strong></td><td>" . $basement . "</td></tr>
<tr style='padding:5px'><td><strong>Notes:</strong></td><td>" . $notes . "</td></tr>";
mail("****#****", "Listing Inquiry", $message, "Content-Type: text/html; charset=ISO-8859-1\r\n");
header('Location: sellRedirect.php');
}
}
}
?>
Thanks!

Answer 1

To check if the field has been sent, you should use isset() function.
For example:
if(isset($_POST['name'])) {
// The field has been sent
} else {
die('Something is wrong!');
}