switch.php

<?php

$switchstate = ""; // Init a variable to keep track of the switched state of the HTML input, this way we can show the state in the actual button.

if($_SERVER["REQUEST_METHOD"]==="POST") { // Look to see if we submitted a form to this page. If we did, then we will execute the code to change the file.

  $file = fopen("textfile.txt", "w"); // Open the file called "textfile.txt" in "write" mode.

  if(@$_POST["updateStatus"]==="on") { // Check the status of the input. Notice in the html at the bottom of the file the name of the checkbox input is the same as within the brackets. (updateStatus)
    // The checkbox is checked.
    $switchstate = "checked"; // Change the variable that holds tge state of the HTML switch so it showes checked and the user can uncheck it.
    fwrite($file,"1"); // Write "1" to the text file, overwriting whatever was in it.
  } else {
    // The checkbox is unchecked.
    $switchstate = ""; // Make sure the HTML switch gets unchecked.
    fwrite($file,"0"); // Writes "0" to the text file, overwriting whatever was in it.
  }
}

$status = file_get_contents("textfile.txt"); // Get the contents of a file called "textfile.txt" saved in the same directory, and save it a variable $status

if($status == 0) {
  // The switch is off
  echo "The switch is off"; // Echo: PHP's version of knowing what to show on the page.
} else {
  // The switch is on
  echo "The switch is on"; // Echo: PHP's version of knowing what to show on the page.
}

?>

<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="POST"> <!-- HTML form who's action is this file, and method is POST. -->
  <input type="checkbox" name="updateStatus" <?php echo $switchstate;?>> <!-- Checkbox. Notice we are echoing (displaying) the switchstate computed above -->
  <input type="submit" value="update">
</form>