How to use PHP isset function

Using $ _POST or $ _GET to retrieve the variables from a form, you may encounter error:
Notice: Undefined index
To avoid this error, use function isset ().

isset — Determine if a variable is set and is not NULL

if (isset($_POST['value'])) {
   // Some instructions if $_POST['value'] exist
} else {
  // $_POST['value'] is not set, do something and raise an error
}

if(isset($_GET['action'];)) {
  $action = $_GET['action'];
} else {
  // ERROR CODE
}

You should use isset() instead of empty(), so you avoid the undefined index error:

$term = $_POST['term'];
if (empty($term)){
   $term = $_GET['term'];
}

change to :

if(!isset($_POST['term'])){
   if (isset($term) && $term ) {
      $term = $_GET['term'];
   }
}

// if the form's submit button is clicked, we need to process the form
if (isset($_POST['submit']))   {
    // make sure the 'id' in the URL is valid
    if (is_numeric($_POST['id']))  {
      // get variables from the URL/form
      $id = $_POST['id'];
      $name = $_POST['name'];
      $username = $_POST['username'];
      $password = $_POST['password'];
      $email = $_POST['email'];

      // check that name,username, password and  email are not empty
      if ($name == '' || $username == '' || $password == '' || $email == '')  {
          // if they are empty, show an error message and display the form
          $error = 'ERROR: Please fill in all required fields!';
      } 
      else {
          // if everything is OK, update the record in the database
          if ($stmt = $mysqli->prepare("UPDATE admin_login SET name = ?, username = ?,  password = ?, email = ?  WHERE id=?"))  {
              $stmt->bind_param("ssssi", $name, $username, $password, $email, $id);
              $stmt->execute();
              $stmt->close();
          }
            // show an error message if the query has an error
          else  {
            echo "ERROR: could not prepare SQL statement.";
          }
          // redirect the user once the form is updated
          header("Location: admin_user.php");
      }
    }
    // if the 'id' variable is not valid, show an error message
    else {
       echo "Error!";
   }
}

PHP isset() with multiple parameters