php - Error when trying to append text to a new line within a text file

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I am using webhosting000 for a sixth-form computing unit. This is my first time using PHP so please forgive any beginner errors, when I try to append $userInput into UserNotes.txt I am receiving an error message.

Notice: Undefined index: userMessage in /storage/ssd3/381/15908381/public_html/index.php on line 34

I have read through the PHP website and W3schools on the use of \n and \r\n to append to a new line but I cant find the source of the problem within my code. What do I need to do to solve this error and be able to save the user's message within the text file on a new line?

<!DOCTYPE html>
<html>
    <body>
        <h1>This Is a Website</h1>
            <?php 
                echo "I wrote this using php";
            ?>
        <br>
        <h2>Your IP Address:</h2>
            <?php
                $user_IP = $_SERVER['REMOTE_ADDR'];
                echo $user_IP;
            ?>
        <br>
        <h2>Your browser Information:</h2>
            <?php
                $browserInfo = $_SERVER['HTTP_USER_AGENT'];
                echo $browserInfo;
            ?>
        <br>
        <h2>Your Screen Information</h2>
            <script>
                var screenWidth = window.screen.width;
                var screenHeight = window.screen.height;
                document.write("Screen width: " + screenWidth + "px" + "<br />");
                document.write("Screen height: " + screenHeight + "px" + "<br />");
            </script>
        <br>
        <h2>Send me a message!</h2>
            <?php
                $file="userNotes.txt";
                if(isset($_POST["submitButton"]))
                {
                    $userInput = $_POST["userMessage\r\n"];
                    file_put_contents($file, $userInput, FILE_APPEND | LOCK_EX);
                }
            ?>
            <form action="" method="post">
                <input type="text" name="userMessage">
                <input type="submit" value="send" name="submitButton">
            </form>
    </body>
</html>

Answer

Solution:

"You are using "userMessage\r\n" as the field name. If you want to add a new line after the field, use $_POST["userMessage"] ."\r\n" ??� Nigel Ren 1 hour ago"

Source