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refreshing page issue

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In the following code I'm using a combo box to select a relay and activate it. The code works but if I refresh the page, the last selected relay is activated again.

How can I reset the page to default to value '0' which has no relay control?



<!DOCTYPE html>
<title>Relay control</title>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type" />
<meta name="apple-mobile-web-app-capable" content="yes" />
<meta name="apple-mobile-web-app-status-bar-style" content="black-translucent" />
<link rel="stylesheet" type="text/css" href="style.css" />
<h2><center>Status y control de relés</center></h2>
$arrleds = array();

  $selected_val = $_POST['sel'];

  if ($selected_val == '1'){
  $cmd = exec("sudo ./trigger.py b 0");
  elseif ($selected_val == '2'){
  $cmd = exec("sudo ./trigger.py b 1");
  elseif ($selected_val == '3'){
  $cmd = exec("sudo ./trigger.py b 2");


  else {}

<table class="center">
 <td><h1>Activación de relé</h1>
  <form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
        <select name='sel' onchange='submit();'>
                <option value='0'>Sin activar</option> <!-- no action -->
                <option value='1'>Secadora 1</option>
                <option value='2'>Secadora 2</option>
                <option value='3'>Lavadora S</option>
 <td><a href="index.php">Home</a>


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@gw1500se is correct, you would use PRG (Post/Redirect/Get). You would not need Sessions in this case.



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