Simple Chat Script

<?php
$chat = "./chatlogs.txt";
$strdata = $_POST['msg'];
$username = $_POST['sn'];
$maxLines = 15;
$message = '';
function nameColor($name){
    return substr(md5($name),0,6);
}

if(!file_exists($chat)){
    file_put_contents($chat, '');
}

$chatLines = explode(PHP_EOL,file_get_contents($chat));
if(($linesOver = (count($chatLines) - $maxLines)) > 0){
    $chatLines = array_slice($chatLines,$linesOver);
}
$chatLines = implode(PHP_EOL,$chatLines);


if(strlen($_POST['submit']) > 0){
    if (strlen(trim($strdata,' .')) > 0 && strlen($username) > 0){
        $chatLines .= '<span style="font-weight: bold;color: ' . nameColor($username) . ';">' . $username . '</span>: ' .  $strdata . "<br />" . PHP_EOL;
        $fh = fopen($chat,'w');
        fwrite($fh, $chatLines);
        fclose($fh);
    }else if(strlen($username) === 0){
        $message =  "<font color=\"FF0000\">You must enter a username!</font>";
    }
}
?>
<html>
    <head>
        <title>Simple Chat Page</title>
    </head>
<body onLoad="document.getElementById('msg').focus();">
    <?php
        echo $chatLines . PHP_EOL . '<hr />' . PHP_EOL . $message . PHP_EOL;
    ?>
    <form method="POST" autocomplete="off">
        <?php
            if(strlen($username) > 0){
                echo '<input type="hidden" name="sn" value="' . $username . '">';
            }else{
                echo 'Name: <input type="text" size="7" name="sn" value="' . $username . '">';
            }
        ?>
        Message: <input type="text" id="msg" name="msg">
        <input type="submit" name="submit" value="Send">
        <input type="submit" name="submit" value="Check for new messages">
    </form>
</body>
</html>