php，从$ _POST搜索字符串的txt并覆盖包含该变量的行

so I'm trying to use only php and html to build a simple comment system. I can Login users and post comments but editing comments does not work.

So i save posted comments in a txt file with the format:

User|commentSignature|commentContent

Where comment signature is a substring of commentContent. Then when the user is logged in i show his comments in a forms textare so he can submit that form and edit them easily.

Here is the form to edit comments:

$line = trim($line);
$commentString= 'Comment';
$arr = explode("|", $line, 3);

// This doesent work
// It is about editing comments
if (isset($_SESSION['user']) and $_SESSION['user'] == $arr[0]) { ?>


    <form action="editComment.php" method="POST">
    <textarea rows="10" cols="30" name="commentContent"> <?php echo "Here is youre old comment: " . $arr[2] ?></textarea>
    <br /> 
    <input type="hidden" name="oldCommentSignature" value= "<?php echo $arr[1]; ?>">

    <input type="submit" value="Edit"> <br />
    </form>

So as you can see i send a hidden value oldCommentSignature to be able to find the comment I shall overwrite.

So far so good i print the $_POST and $_SESSION values and they get sent but my overwriting method in editComment.php is not working.

<?php   session_start();

print_r($_POST);    print_r($_SESSION);

$handle = fopen ( "save.txt", "r+" );   if ($handle &&
isset($_SESSION['user']) && $_POST) {
        while ( ($line = fgets ( $handle )) !== false ) {
            $line = trim($line);            $arr = explode("|", $line, 3);
            //$compareValue = $arryOfLine[1];

            if ($_POST["oldCommentSignature"] == $arr[1]) {

                $newCommentSignature = substr($_POST ["commentContent"], 0, 9);
                $nameAndTimeAndContent = $_SESSION['user'] . "|" .
                $newCommentSignature . "|" . $_POST ["commentContent"] . "
";  

                 fwrite($handle, $nameAndTimeAndContent);
                 break;
            }       
      }
}   fclose ( $handle );

?>

Sometimes nothing is happening and sometimes i get the message

Notice: Undefined offset: 1 in /Applications/XAMPP/xamppfiles/htdocs/tr/editComment.php on line 16*