Failure message doesn't display when invalid PHP form is submitted

Question

I'm having some difficulty with a PHP form, the problem being that my confirmation message displays even when the form fields have not been validated.

If the form has not been validated, I would like to display an alternative message that explains to the user that something went wrong and their message was not sent.

Unfortunately, my understanding of PHP is only as good as the tutorials I've taken in trying to put this form together so I'm a bit stuck! Would someone be kind enough to explain to me what I am doing wrong?

Here's the code:

<?php
ini_set('display_errors', 'On');

//Defines error variables and sets to empty
$senderErr = $senderEmailErr = $messageErr = "";
//Defines text entry variables and sets to empty
$sender = $senderEmail = $message = "";
//Defines failure/confirmation messages and sets to empty
$thankYou = "";
$formFail = "";

function test_input($data) {
   $data = trim($data);
   $data = stripslashes($data);
   $data = htmlspecialchars($data);
   return $data;
}

if ($_SERVER["REQUEST_METHOD"] == "POST") {
   if (empty($_POST["sender"])) {
     $senderErr = "Name is required";
   } else {
     $sender = test_input($_POST["sender"]);
     // Checks that name only contains letters and whitespace
     if (!preg_match("/^[a-zA-Z ]*$/",$sender)) {
       $senderErr = "Only letters and white space allowed";
     }
   }

   if (empty($_POST["senderEmail"])) {
     $senderEmailErr = "Email is required";
   } else {
     $senderEmail = test_input($_POST["senderEmail"]);
     // Checks if e-mail address syntax is valid
     if (!preg_match("/([\w\-]+\@[\w\-]+\.[\w\-]+)/",$senderEmail)) {
       $senderEmailErr = "Invalid email format";
     }
   }

   if (empty($_POST["message"])) {
     $messageErr = "You haven't written anything";
   } else {
     $message = test_input($_POST["message"]);
     }

if($_POST["submit"]) {
    $recipient="me@email.com";
    $subject="enquiry";
    $sender=$_POST["sender"];
    $senderEmail=$_POST["senderEmail"];
    $message=$_POST["message"];

    $mailBody="Name: $sender\nEmail: $senderEmail\n\n$message";

    mail($recipient, $subject, $mailBody, "From: $sender <$senderEmail>");

    $thankYou = "Thanks for your message. We'll get back to you shortly";
    }
    else {
        $formFail = "Oops. Something went wrong. Please fill out the required fields.";
    }
}
?>

I think it has something to do with the fact that I'm not setting the right conditions for failure. Problem is, I'm not quite sure where to start. Apologies for being a noob. Help appreciated. Thanks.

Edit - here's the HTML in response to Fred's question about whether the elements are named:

    <form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">

    <label>Name:</label></br>
    <input type="text" name="sender"></br>
    <span class="error">* <?php echo $senderErr;?></span>
    </br>

    <label>Email address:</label></br>
    <input type="text" name="senderEmail"></br>
    <span class="error">* <?php echo $senderEmailErr;?></span>
    </br>

    <label>Message:</label></br>
    <textarea rows="5" cols="20" name="message"></textarea></br>
    <span class="error">* <?php echo $messageErr;?></span>

    <input type="submit" name="submit" value="submit"></br>
</form>

<span><?php echo $thankYou;?></span>
<span><?php echo $formFail;?></span>

Answer 1

Replacing if($_POST["submit"]) with if(($_POST["submit"]) && !empty($_POST["sender"]) && !empty($_POST["senderEmail"]) && !empty($_POST["message"])) will fix the problem.

Using PHP's empty() function, ensures that if something is left "empty" and used in a conditional statement, will not execute until all conditions are met.

All being the && (logical) operator.