<form method="post" action="">
  <input type="checkbox" name="checkbox[]" value="value1"> Option 1
  <input type="checkbox" name="checkbox[]" value="value2"> Option 2
  <input type="checkbox" name="checkbox[]" value="value3"> Option 3
  <input type="submit" value="Submit">
</form>

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
  if (isset($_POST['checkbox'])) {
    $checkbox_values = $_POST['checkbox'];
    foreach ($checkbox_values as $value) {
      echo $value . "<br>";
    }
  }
}
?>