Header Error In Plugin

Hi there i am new to this WordPress Development…Here i am making a Plug in that will run a Script with some Parameters which can be set by User and that script will run in wp_footer apart from that i am saving user data in a simple text file which is created when the user submit the data and that file is saved with the login name of user and accomplished it i am successfully making a user submit the values and file is also been created but when i logout it is not making me logout and either when i try to login from other browser it also give me header errors here i will post the code below:

plugin index.php:

<?php
/*
Plugin Name: Dhudora
Version: 0.1
Description: Advertisment Plugin
Author: Fahim Mahmood Mir
Author URI: https://www.prelovac.com/vladimir
Plugin URI: https://www.prelovac.com/vladimir/wordpress-plugins/
wp-digg-this
*/
?>
<!--<script type="text/javascript">

function formValidator(forme)
{

	var limit = document.getElementById('txtlimit').value;
	var seck = document.getElementById('txtkey').value;
	var interval = document.getElementById('txtinterval').value;

}

</script>-->

<?php
add_action('admin_menu', 'basicPluginMenu');
function basicPluginMenu() {
$appName = 'Dhundora';
$appID = 'basic-plugin';
add_menu_page($appName, $appName, 'administrator', $appID, 'pluginAdminScreen');
}
function pluginAdminScreen() {

	  global $current_user;
      get_currentuserinfo();

       $userlog=  $current_user->user_login ;

	echo '<h1> Welcome to Dhundora Advertising Script</h1>';
	?>
    This is the Plugin that will be helping the Dhundora to Successfully launch the Script In this Plugin of WordPress<br>

<form id="forme" action="../wp-content/plugins/dhundora/process-file.php"  method="get">
<br /><br />
<span name="login-subheading">Secrect Key</span><br />
<input  id="txtkey" name="userkey" type="text" style="width:300px" /><br />
<br />

<input type="hidden" id="txtuser" name="username" value ='<?php echo $userlog; ?>' /><br />

<span name="login-subheading">Limit</span><br />
<input id="txtlimit" name="userlimit" style="width:300px" type="text" />&nbsp;

<br /><br />

<span name="login-subheading">Interval</span><br />
<input id="txtinterval" name="userinterval" type="text" style="width:300px" />&nbsp;<br /><br />

<input id="login-submit" name="Continue" type="submit" value="Register" style="padding:8px"/>
</form>
<div id="result">
<?php
$data = array();
$myFile = "../wp-content/plugins/dhundora/".$userlog.".txt";
$fh = fopen($myFile, 'a+') or die("No Settings Saved");
$i = -1;
//Output a line of the file until the end is reached
while(!feof($fh))
  {
	  $i = $i+1;
  $data[$i] = fgets($fh);
  }
fclose($fh);

echo  '<br /><br />'.'<input type="text" readonly="readonly" value =  '.$data[0]. ' /><br />';
echo  '<br /><br />'.'<input type="text" readonly="readonly" value =  '.$data[1]. ' /><br />';
echo  '<br /><br />'.'<input type="text" readonly="readonly" value =  '.$data[2]. ' /><br />';

?>
</div>

<?php

}

?>

and the form submittion file process-file.php

<?php

/*$userlim =  $_GET['userlimit']."\n";
$userint =  $_GET['userinterval']."\n";
$userkey = $_GET['userkey']."\n";
*/
$myFile = $_GET['username'].".txt";
$fh = fopen($myFile, 'w+') or die("can't open file");

$stringData = $userkey = $_GET['userkey']."\n".$_GET['userlimit']."\n".$_GET['userinterval']."\n";
fwrite($fh, $stringData);
fclose($fh);

?>
Your Settings are Saved <a href="javascript:void()" onclick="window.history.back();" > Go to Back</a><br>

can anyone tell me why this thing giving header error ??