PHP - How To Avoid Duplicate/multiple Sql Queries By Refreshing Page?

<?php 

// Get required login info
include "/path/to/login/info/file.php"; // Get required login info - changed for this post.

$db = new mysqli('localhost', $username, $password, $database); // Connect to DB using required login info
if($db->connect_errno > 0){
    die('Unable to connect to database [' . $db->connect_error . ']');
}

unset($username);// put these variables back to null
unset($password);// put these variables back to null
unset($database);// put these variables back to null


//query db
$sql = <<<SQL
    SELECT *
    FROM `ft_form_7`
	WHERE DATE(class_start_date) >= CURDATE()
	OR DATE(class_end_date) >= CURDATE()
	ORDER BY class_start_date ASC
SQL;

if(!$result = $db->query($sql)){ // if there is an error in running the query, show error message.
    die('There was an error running the query [' . $db->error . ']');
}

while($row = $result->fetch_assoc()){
	// Get start date information
	$start_date = $row['class_start_date']; // Get event_start_date for conversion and call it $start_date
	$start_date_formatted = date("l M d, Y", strtotime($start_date)); // Convert start_date 
	$end_date = $row['class_end_date']; // Get event_end_date for conversion and call it $start_date
	$end_date_formatted = date("M d, Y", strtotime($end_date)); // Convert start_date
	
	// Get time information.
	$start_time = $row['class_start_time']; // Get event_start_time for conversion and call it $start_time
	$start_time_formatted = date("h:i A", strtotime($start_time)); // Convert start_time 
	$end_time = $row['class_end_time']; // Get event_end_time for conversion and call it $end_time
	$end_time_formatted = date("h:i A", strtotime($end_time)); // Convert end_time 
	
	// echo information...
	    echo "<h2>" , $row['class_name'],"</h2>" ; // echo event name
	    echo "<p><strong>",$start_date_formatted; // echo the start date
		if (empty($start_time)) { echo ''; } else { echo " (", $start_time; } // echo start time
		if (empty($end_date)) { echo ''; } else { echo " -","<br />", $end_date_formatted; } // echo end date
		if (empty($end_time)) { echo ')'; } else { echo " - ", $end_time, ")"; } // echo end time
	// if there is no start time, echo nothing. (otherwise it seems to echo 4pm). If it does contain a time, echo the time.
	echo "</strong><br />";

	$chef = $row['Instructor'];
	global $chef;
	
	if ($chef != NULL) {
		require ('instructors.php'); }
	echo $row['class_description'], "<br />";
	echo "<strong>" , $row['type'], "&nbsp;-&nbsp;Cost: &#36;",$row['cost'] , " - #" , $row['course_number'] , "</strong><br />" , "</p>"; // echo class type and cost
}

$db->close();
$result->free();

?>

<?php

include "/path/to/login/info/file.php"; // Get required login info - changed for this post.

$db_instructors = new mysqli('localhost', $username, $password, $database); // Connect to DB using required login info
if($db_instructors->connect_errno > 0){
    die('Unable to connect to database [' . $db_instructors->connect_error . ']');
}

unset($username);// put these variables back to null
unset($password);// put these variables back to null
unset($database);// put these variables back to null

//query db
$sql_instructors = <<<SQL
    SELECT *
    FROM ft_form_8
    WHERE chef_name = '$chef'
SQL;

if(!$result_instructors = $db_instructors->query($sql_instructors)) { // if there is an error in running the query, show error message.
    die('There was an error running the query [' . $db_instructors->error . ']');
}

	while($row_instructors = $result_instructors->fetch_assoc()) {
	$chef_full_name = $row_instructors['chef_name'];
	$chef_id = $row_instructors['submission_id'];
	$full_bio = $row_instructors['full_bio'];
	echo "<a href=\"#\" class=\"clickme\">" , $chef_full_name , "</a>";
	echo "<div class=\"box\">";
	echo $full_bio , "</div>";
}


$db_instructors->close();
$result_instructors->free();

?>

<h1>Do Add a Message to the MySQL Database</h1>
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="POST">
    <textarea name="message"></textarea>
    <br/>
    <input type="submit" name="submit"/>
</form>

<?php
// The Connection to the Database
// Taken Out

?>

<?php
// To insert the text data into the MySQL database.
if(isset($_POST['submit'])){

$tqs = "INSERT INTO messages (`message`) VALUES ('{$_POST['message']}')";

$tqr = mysqli_query($dbc, $tqs) or die(mysqli_error($dbc));

}

?>

<?php
// To select the text data from the MySQL database.
$tqs = "SELECT * FROM messages";
$tqr = mysqli_query($dbc, $tqs);


// To print out the text data inside of table on the page.
echo "<h1>This Is Where the Messages Gets Printed on Screen</h1>";
echo "<table><tr><td>ID</td><td>The Message</td></tr>";

while($row = mysqli_fetch_assoc($tqr)){

    echo "<tr><td>" . $row['id'] . "</td><td>" . $row['message'] . "</td></tr>";

}

echo "</table>";

?>

$error=array();
$sql = 'INSERT INTO staff(username, email, phone)
VALUES (?, ?, ?)';
$stmt = $conn->stmt_init();
$stmt = $conn->prepare($sql);
// bind parameters and insert the details into the database
$stmt->bind_param('sss', $username, $email, $phone);
$stmt->execute();
if ($stmt->errno == 1062) {
$errors[] = "One of the fields is already in use.";
}

Hi,

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?

TIA

<!DOCTYPE html>
<html>
<head>
<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" />
</head>
<body>
<h2><center>Status y control de relés</center></h2>
<?php
$arrleds = array();

if ($_SERVER["REQUEST_METHOD"] == "POST") {
  $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">
<tr>
 <td></td>
 <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>
                ....
        </select>
  </form>
 </td>
 <td></td>
</tr>
<tr>
 <td></td>
 <td><a href="index.php">Home</a>
 <td></td>
</tr>
</table>
</body>
</html>