Create a new file named send_email.php in the same directory as your HTML file.

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    // Get form data
    $name = htmlspecialchars($_POST['name']);
    $email = htmlspecialchars($_POST['email']);
    $subject = htmlspecialchars($_POST['subject']);
    $message = htmlspecialchars($_POST['message']);

    // Your email address where you want to receive the emails
    $recipient_email = "[email protected]"; // *** REPLACE WITH YOUR EMAIL ADDRESS ***

    // Email subject
    $email_subject = "New Contact Form Submission: " . $subject;

    // Email body
    $email_body = "You have received a new message from your website contact form.\n\n";
    $email_body .= "Name: " . $name . "\n";
    $email_body .= "Email: " . $email . "\n";
    $email_body .= "Message:\n" . $message . "\n";

    // Headers for the email
    $headers = "From: " . $name . " <" . $email . ">\r\n";
    $headers .= "Reply-To: " . $email . "\r\n";
    $headers .= "X-Mailer: PHP/" . phpversion();

    // Send the email
    if (mail($recipient_email, $email_subject, $email_body, $headers)) {
        // Redirect to a thank you page or display a success message
        echo "<p>Thank you for your message. It has been sent successfully!</p>";
        // Or redirect: header("Location: thank_you.html");
    } else {
        // Display an error message
        echo "<p>An error occurred while sending your message. Please try again later.</p>";
    }
} else {
    // If accessed directly without POST request
    echo "<p>Invalid request.</p>";
}
?>

Note: Emails sent via the mail() function can sometimes be flagged as spam.