YA: Simple Comment System

<?php
/*
    Responce to Yahoo Question:
    http://answers.yahoo.com/question/index?qid=20111115110533AAkaHed
*/

$textFile = 'savedMessages.txt'; // Rename to anything you want;
$output = ''; // Holds any message you want to show the user.
$messages = ''; // Holds the saved messages from the text file.

if($_SERVER['REQUEST_METHOD'] === 'POST'){
    $text = trim( strip_tags($_POST['user_input'],'<a><b><i>') ); // Strips any HTML Markup from the input, but allows for tags a, b and i. Allowing for links and minor styling.
    if(strlen($text) > 0){ // Makes sure there's some text input.
        $saveString = "\t\t" . '<div style="margin-bottom: 8px;"><span style="font-variant: italic;font-size: 12px;">' . date("M j Y H:i:s") . '</span><br>' .
                            '<span>' . $text . '</span></div>' . PHP_EOL . PHP_EOL; // Rough template of how the input will be formatted to appear in the page. PHP_EOL is a PHP constant, meaning EndOfLine character, depending on the system.
        $fh = fopen($textFile,'a+') or die('Could not open file: <b>' . $textFile . '</b> to write to.'); // Open the text file to save the new string to. If PHP cannot open the file, then die(message)
        fwrite($fh, $saveString) or die('Could not write new string to file.'); // Write the saveString variable to the file, or die(message);
        fclose($fh) or die('Could not close file.'); // Close the file or die(message);
        $output = 'Thank you, your message has been saved.';
    }else{
        $output = 'Invalid Input to Save';
    }
}
if(file_exists($textFile)){ // Checks to see if the file exists.
    $messages = file_get_contents($textFile);    // Gets the contents of the file
}else{
    $messages = '<span style="font-variant: italic;font-weight: bold;">No Saved Messages</span>'; // Default message to display when there is no Messages file.
}

?>
<html>
    <head>
        <title>Simple Message Saving Script</title>
    </head>
<body>

    <div id="output">
        <span><?php echo $output; ?></span>
    </div>
    <div id="formContainer">
        <span>Allowed Tags: &lt;a&gt;,&lt;b&gt; and &lt;i&gt;.</span>
        <br>
        <form method="post" action="">
            <textarea name="user_input" style="width: 500px;height: 300px;"></textarea>
            <br>
            <input type="submit" value="Save" style="position: relative;left: 450px;">
        </form>
    </div>
    <div id="msgsContainer">
        <?php echo $messages; ?>
    </div>
</body>
</html>