<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
# action defines where the form contents are sent when the form is submitted
# htmlspecialchars() is used here to convert specific HTML characters to their HTML entity names .e.g. > will be converted to >
# $_SERVER is an array with entries filled in at PHP run time; the PHP_SELF key contains the filename of the PHP script that is being executed 
# method determines how the form’s contents is submited
# POST means that the form’s content is sent as part of the HTTP request’s body 
# trim() function use to remove white space from a single variable start and end point 
# php filter to use validate and sanitize data . Validating data = Determine if the data is in proper form.Sanitizing data = Remove any illegal character from the data.

               // check submit button is clicked
  if ($_SERVER['REQUEST_METHOD'] == "POST" && isset($_POST['submit'])) {
    $name = trim($_POST['name']);
    $email = trim($_POST['email']);
    $amount= trim($_POST["amount"]);
                // if fill input field
     if (!empty($name) && !empty($email)) {
         // validate String 
         if (!filter_var($name,FILTER_SANITIZE_STRING)) {
             $nameerr = " Your Text Is Not Valid  ";
         // Validate Email
        if (!filter_var($email,FILTER_VALIDATE_EMAIL)) {
            $emailerr = " Please Provide A Valid Email Address ";

        if (!is_numeric($amount)) {
            $amounterr =" Please Give Valid Number ";

  if (empty($name)) {
      $nameerr = "Name Is Required";

  if (empty($email)) {
      $emailerr = " Please Fill Email Field ";

  if (empty($amount)) {
    $amounterr = " Please Fill Amount Field ";
    <form action="<?php echo htmlspecialchars ($_SERVER['PHP_SELF']);?>" method="post">
        <label for="name"> Name </label> <input type="text" name="name" id="name">         <?php echo $nameerr;?> 
        <label for="email"> Email </label> <input type="text" name="email" id="email">  <?php echo $emailerr;?>  <br>
        <label for="amount"> Amount </label> <input type="text" name="amount" id="amount">  <?php echo $amounterr;?>  <br>

        <button type="submit" name ="submit">Submit</button>