Notice: Uninitialized string offset: 1

Hallo,

Ik krijg steeds de melding: Notice: Uninitialized string offset: 1 in C:\xampp\htdocs\status.php on line 77
Ik krijg met geen mogelijkheid de melding weg.
Hopelijk kunnen jullie mij helpen.

GrtZ Evert

Hier is het script.

<meta http-equiv="Refresh" content="10;URL=status.php">

<?php



// uitlezen syteemfunctie

$File = "System.txt";
$fh = fopen($File, 'r');
$Dt = fgets($fh);
fclose($fh);


echo "<font color='white'>";
echo "<table border=\"0\" align=\"left\">";
echo "<tr><th>IO-nr</th>";
echo "<th> Status </th></tr>";

for ($counter = 0 ; $counter <= 15 ; $counter += 1)
{


// *****************************************op onderstaande blijft ie hangen *******************;

$Ps = $Data[$counter];

// **********************************************************************************************;




$c=$counter+1;

$File='poort' .$c. '.txt';
$fh = fopen($File, 'r');
$Data = fgets($fh);
fclose($fh);



if ( $Ps == 1 )

{

$onoff='<img src="../images/img0007.png" id="Image1" alt="" border="0" style="width:20px;height:20px">'."<br />";
}
else
{

$onoff='<img src="../images/img0008.png" id="Image1" alt="" border="0" style="width:20px;height:20px">'."<br />";

}

If ($Dt == 0)
{

$onoff='<img src="../images/img0009.png" id="Image1" alt="" border="0" style="width:20px;height:20px">'."<br />";
}

echo "<tr><td>";
echo chr(32);
echo $counter+1;
echo "</td><td>";
echo chr(32);
echo $onoff;
echo "</td></tr>";

}
echo "</table>";

?>