php Limit query pdo with button next

whats the wrong in my code .. i try to click next button to display the next record in database .. but when i clicked nothing happen.

this is my function to fetch record

public function getBooks()
{
     $limit = 1;
     $start=0;

    //SELECT loginUser.username, Library.nameOfBook FROM loginUser JOIN userBook JOIN Library ON userBook.user_id = loginUser.id AND userBook.book_id = Library.id WHERE loginUser.username="loay";
    $query = "SELECT Library.nameOfBook FROM loginUser JOIN userBook JOIN Library ON userBook.user_id = loginUser.id AND userBook.book_id = Library.id WHERE loginUser.username=:username LIMIT $start, $limit";
    $statment = $this->db->prepare($query);
    $statment->execute([
        ':username' => $this->username
        //,':start' => $start, ':limit' => $limit
    ]);
    $result = $statment->fetchAll();

   echo "<table border='1'>
    <tr>
    <th>Books</th>
    </tr>";
    foreach($result as $row){
        echo "<tr>";
        echo "<td>" . $row['nameOfBook'] . "</td>";
        echo "</tr>";
    }
    echo "</table>";

}

And this is my code in index.php

  $object->getBooks();

  if( isset($_POST['next'])){
    $start +=1;
}

and this is my form code

<form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
  Name: <input type="text" name="user" value="<?php echo $user;?>">
  <span class="error">* <?php echo $nameErr;?></span>
  <br><br>
  password: <input type="password" name="pass" value="<?php echo $password;?>" >
  <span class="error">* <?php echo $passwordErr;?></span>
  <br><br>
  <input type="submit" name="submit" value="Login">
  <br><br>

 
  <button type="button" name="button" onclick="window.location.href='/ooRigester.php'">SignUp</button>
  <br><br>
  <input type='submit' name='next' value='next' method="post"><br>
</form>

this code after form

<?php
include_once('User.php');

if(isset($_POST['submit'])){

$username = $_POST["user"];
$password = $_POST["pass"];

$object = new User();
$object->username= $username;
$object->Password=$password;
if( $object->isAuthenticated() ){

  echo "User Verified . <br><br>";
  $object->getBooks();

  if( isset($_POST['next'])){
    $start +=1;
    $object->getBooks($start); 
}

}

else{

  echo "Wrong User Name Or Password . <br>";
}}


?>

and this before form

<!DOCTYPE HTML>
<html>
<head>
<style>
.error {color: #FF0000;}
</style>
<title>Login Form</title>
</head>
<body>
<?php
/*$str = "Hello";
echo password_hash($str,1);*/
// define variables and set to empty values
$nameErr = "";
$passwordErr = "";
$user = "";
$password = "";

if ($_SERVER["REQUEST_METHOD"] == "POST") {
  if (empty($_POST["user"])) {
    $nameErr = "Name is required";
  } else {
    $user = test_input($_POST["user"]);
    // check if name only contains letters and whitespace
    if (!preg_match("/^[a-zA-Z ]*$/",$user)) {
      $nameErr = "Only letters and white space allowed";
    }
  }
  if (empty($_POST["pass"])) {
    $passwordErr = "Password is required";
  } else {
    $password = test_input($_POST["pass"]);
  }
  
}

function test_input($data) {
  $data = trim($data);
  $data = stripslashes($data);
  $data = htmlspecialchars($data);
  return $data;
}
?>

Solution

In Index.php - you're not setting $start value properly. The below code sets $start value from $_POST['start'] which if non-existent $start defaults to 0.

And you're also not accepting form submission properly. I modified the code to accept both submit types (next and login) by doing isset() checks on their $_POST variables.

Use:

<?php
include_once('User.php');

if(isset($_POST['submit']) OR isset($_POST['next'])){
    $username = $_POST["user"];
    $password = $_POST["pass"];

    $object = new User();
    $object->username= $username;
    $object->Password=$password;
    if( $object->isAuthenticated() ){

      echo "User Verified . <br><br>";

      $start = 0;
      if(isset($_POST['start'])){
         $start = $_POST['start'];
      }

      $object->getBooks($start);

    }
}
else{
  echo "Wrong User Name Or Password . <br>";
}


?>

In your form code, you have to add a hidden input which holds the offset value of the next database record.

Use this code:

<form method="post" action="">
  Name: <input type="text" name="user" value="<?php echo $user;?>">
  <span class="error">* <?php echo $nameErr;?></span>
  <br><br>
  password: <input type="password" name="pass" value="<?php echo $password;?>" >
  <span class="error">* <?php echo $passwordErr;?></span>
  <br><br>
  <input type="submit" name="submit" value="Login">
  <br><br>

  <input type="hidden" value="<?php echo (intval(isset($_POST['start'])?$_POST['start']:0)+1);?>" name="start"/>
  <button type="button" name="button" onclick="window.location.href='/ooRigester.php'">SignUp</button>
  <br><br>
  <input type='submit' name='next' value='next' method="post"><br>
</form>

Your class method getBooks() has to have $start parameter so it knows which record is next to be shown.

Try:

public function getBooks($start = 0)
{
     $limit = 1;

    //SELECT loginUser.username, Library.nameOfBook FROM loginUser JOIN userBook JOIN Library ON userBook.user_id = loginUser.id AND userBook.book_id = Library.id WHERE loginUser.username="loay";
    $query = "SELECT Library.nameOfBook FROM loginUser JOIN userBook JOIN Library ON userBook.user_id = loginUser.id AND userBook.book_id = Library.id WHERE loginUser.username=:username LIMIT $start, $limit";
    $statment = $this->db->prepare($query);
    $statment->execute([
        ':username' => $this->username
        //,':start' => $start, ':limit' => $limit
    ]);
    $result = $statment->fetchAll();

   echo "<table border='1'>
    <tr>
    <th>Books</th>
    </tr>";
    foreach($result as $row){
        echo "<tr>";
        echo "<td>" . $row['nameOfBook'] . "</td>";
        echo "</tr>";
    }
    echo "</table>";
}