php email automatically displays notification even if i just opened my site - php

Question

I'm just beginner when it comes to coding, don't judge.
I wanted to display a notification after I clicked the submit button or to display a notification if there is something wrong with the data I input with the form. But it seems that the notification was already displayed even if I just opened the file.
Here's the code:
<?php
// Only process POST reqeusts.
if ($_SERVER["REQUEST_METHOD"] == "POST") {
// Get the form fields and remove whitespace.
$name = strip_tags(trim($_POST["name"]));
$name = str_replace(array("\r","\n"),array(" "," "),$name);
$email = filter_var(trim($_POST["email"]), FILTER_SANITIZE_EMAIL);
$message = trim($_POST["message"]);
// Check that data was sent to the mailer.
if ( empty($name) OR empty($message) OR !filter_var($email, FILTER_VALIDATE_EMAIL)) {
// Set a 400 (bad request) response code and exit.
http_response_code(400);
echo "Oops! There was a problem with your submission. Please complete the form and try again.";
exit;
}
// Set the recipient email address.
// FIXME: Update this to your desired email address.
$recipient = "myemail#gmail.com";
// Set the email subject.
$subject = "Customer Inquiry";
// Build the email content.
$email_content = "Name: $name\n";
$email_content .= "Email: $email\n\n";
$email_content .= "Message:\n$message\n";
// Build the email headers.
$email_headers = "From: $name <$email>";
// Send the email.
if (mail($recipient, $subject, $email_content, $email_headers)) {
// Set a 200 (okay) response code.
http_response_code(200);
echo " Thank You! Your message has been sent.";
} else {
// Set a 500 (internal server error) response code.
http_response_code(500);
echo "Oops! Something went wrong and we couldn't send your message.";
}
} else {
// Not a POST request, set a 403 (forbidden) response code.
http_response_code(403);
echo "There was a problem with your submission, please try again.";
}
?>
Here's the form:
<div class="column">
<h3>Get in Touch</h3>
<form action="#" target="myIframe" id="contactUs" method="post">
<div class="field half first">
<label for="name">Name</label>
<input name="name" id="name" type="text" placeholder="Name">
</div>
<div class="field half">
<label for="email">Email</label>
<input name="email" id="email" type="email" placeholder="Email">
</div>
<div class="field">
<label for="message">Message</label>
<textarea name="message" id="message" rows="6" placeholder="Message"></textarea>
</div>
<input name="submit" type="submit" value="Send Message">
</form>
<?php include "mailer.php"?>
</div>

Answer 1

You're getting an error because of the else block at the end. When you're displaying the form it's a GET request, not POST, so the if condition is false. That shouldn't be treated as an error, you should just display the form when that happens.
Get rid of
else {
// Not a POST request, set a 403 (forbidden) response code.
http_response_code(403);
echo "There was a problem with your submission, please try again.";
}

Answer 2

Instead of
if ($_SERVER["REQUEST_METHOD"] == "POST") {
You should have
if ($_SERVER["REQUEST_METHOD"] == "POST" && isset($_POST['submit'])) {
isset($_POST['submit']) checks whether the form is submitted or not.