php undefined index with isset $_POST

trying to create a chat and keep getting Undefined index i've tried adding ? $_POST['chat'] : null; but doesn't work

Notice: Undefined index: chat in /Applications/MAMP/htdocs/chat/chat.php on line 8

Line 8:

$sent = $_POST['chat'];

the variable is used here:

if (isset($_POST['chat'])) {
    if (!empty($sent)) {
        fwrite($myfile, $first.': '.$txt.'=');
        fclose($myfile);
    } else if (empty($sent)) {
        if(isset($_POST['chat'])){
            echo 'Cant send an empty message','<br />';
        }
    }
}

HTML:

<body>
    <iframe id='reload' src='refresh.php'>
        <fieldset class="field">
                <div id="list"><p><?php
                    $filename = 'chat.txt';
                    $handle = fopen($filename, 'r');

                    $detain = fread($handle, filesize($filename));

                    $chat_array = explode('=', $detain);

                    foreach($chat_array as $chat) {
                        echo $chat.'<br />';
                    }
                    ?></p></div>
        </fieldset>
    </iframe>
    <form action="chat.php" method="POST">
        <input type="text" name="chat" class="textbox">
        <input type="submit" value="Send" class="button">
    </form>
</body>

Variables:

    $sent = $_POST['chat'];
    $myfile = fopen("chat.txt", 'a') or die("Unable to open file!");
    $txt = ($sent."\n");
    $first = getuserfield('username');
    $active = ($first.":".$ip_addr);
    $activef = fopen("ip-user.txt", 'a');
    $myFile = "domains/domain_list.txt";

Edit: This is not a duplicate because this is for a very specific piece of code, i've also already used empty and I would not like to ignore the problem because it is a possible cause of another problem.

Thank you.

Solution

Use this code :

<?php 
$sent = '';
if(isset($_POST['chat'])) 
{
    $sent = $_POST['chat'];
    if (!empty($sent))
    {
        $txt = ($sent."\n");
        fwrite($myfile, $first.': '.$txt.'=');
        fclose($myfile);
    } 
    else 
    {
        echo 'Cant send an empty message','<br />';
    }
}
?>