How to return any kind of data from PHP Script Page to HTML Page without using JQuery or AJAX?

Form action tag is there for that action. Lets see on an example. here we have our form main_form.php;

<form action="validation.php" method="post">
<h2>Form</h2>
<span class="error">* required field.</span>
Name:
<input name="name" type="text" value="">
<span class="error">* <?php echo $nameError;?></span>
E-mail:
<input name="email" type="text" value="">
<span class="error">* <?php echo $emailError;?></span>
Gender:
<input name="gender" type="radio" value="female">Female
<input name="gender" type="radio" value="male">Male
<span class="error">*<?php echo $genderError;?></span>
Website:
<input name="website" type="text" value="">
<span class="error"><?php echo $websiteError;?></span>
Comment:
<textarea cols="40" name="comment" rows="5">
</textarea>
<input  name="submit" type="submit" value="Submit">
</form>

Now lets see how we validate on validation.php;

<?php
// Initialize variables to null.
$nameError ="";
$emailError ="";
$genderError ="";
$websiteError ="";
// On submitting form below function will execute.
if(isset($_POST['submit'])){
if (empty($_POST["name"])) {
$nameError = "Name is required";
} else {
$name = test_input($_POST["name"]);
// check name only contains letters and whitespace
if (!preg_match("/^[a-zA-Z ]*$/",$name)) {
$nameError = "Only letters and white space allowed";
}
}
if (empty($_POST["email"])) {
$emailError = "Email is required";
} else {
$email = test_input($_POST["email"]);
// check if e-mail address syntax is valid or not
if (!preg_match("/([\w\-]+\@[\w\-]+\.[\w\-]+)/",$email)) {
$emailError = "Invalid email format";
}
}
if (empty($_POST["website"])) {
$website = "";
} else {
$website = test_input($_POST["website"]);
// check address syntax is valid or not(this regular expression also allows dashes in the URL)
if (!preg_match("/\b(?:(?:https?|ftp):\/\/|www\.)[-a-z0-9+&@#\/%?=~_|!:,.;]*[-a-z0-9+&@#\/%=~_|]/i",$website)) {
$websiteError = "Invalid URL";
}
}
if (empty($_POST["comment"])) {
$comment = "";
} else {
$comment = test_input($_POST["comment"]);
}
if (empty($_POST["gender"])) {
$genderError = "Gender is required";
} else {
$gender = test_input($_POST["gender"]);
}
}
function test_input($data) {
$data = trim($data);
$data = stripslashes($data);
$data = htmlspecialchars($data);
return $data;
}
//php code ends here
?>

If there is an error this script will return error to the our main_form.php.For errors use css to show them in red like below error.css;

.error{
color:red
}

TL;DR: Set form action to validation.php. Keep it simple.

To start and set a session variabl :

session_start(); //at top of page

$_SESSION['name_of_field'] = $_POST['name_of_field']; //do checks on the post data!

To use a session varible :

session_start(); //at top of page

$my_new_value = $_SESSION['name_of_field'];

you will get data in varible.

Or you can send data via URL
Write this in php

 header('Location: http://yoursite.com/page2.php?name='.$name.'&email='.$email);

and get

$name =  $_GET['name'];
$email =  $_GET['email'];

You may also try

Rename your main.html to main.php then include validate.php above your form tag to display errors above form. Thus, your form action would be self or blank

<?php include "validate.php"; ?>

<form action ="" method ="post"... >

But if there are no errors, then from validate.php you can redirect to the index.php for user confirmation using

header('Location: index.php?name='.$name);

The value of name can be accessed using $_GET['name'];