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using ajax to pass string from php to javascript

So I have a piece of javascript code in an html document that responds to a button click. I want a new url to open, and if I specify the link in javascript as I've done below, everything works fine.
<input type="submit" id="submitbtn" value="Purchase Module"/>
<script type="text/javascript">
document.getElementById("submitbtn").addEventListener("click",handle_click);
function handle_click() {
var link;
link="http://www.google.com";
window.location.href=link;
}
</script>
Problem is I want to hide the real link on the server side as it includes a username:password. The php script below calls a function (not shown) that generates the link (a string). This also works fine.
<?php
$link=get_page_link();
?>
I want to pass the link string to the javascript and have tried various iterations of
link=<?php echo $link ;?> to no avail. As I understand it you can't pass strings this way and you need to use ajax. That's where I'm stuck. Seems like I need a $_POST on the php side and a $_GET on the java side, but not sure on the specifics. Any help would be appreciated. Thanks.

Pass Javascript variable to another page via PHP Post

I am having two php pages:
page 1:
<form class="form-horizontal" role="form" method="post" action="Page2.php">
<button id="place-order" class="btn btn-lg btn-success">Place Order</button>
<div id="ajax-loader" style="display:none;"><img src="images/ajax-loader.gif" /></div>
</form>
<script>
var id = Math.random();
$(document).ready(function() {
$('#place-order').on('click', function() {
$(this).hide();
$('#ajax-loader').show();
});
});
</script>
As on form, it redirects to Page2.php, I want to pass the Javascript variable "id" from Page1 to receive it in Page2.
I have tried using cookies, but need an alternative approach.
I am not understanding the transistion from PHP to JS and vice-versa. Help is appreciated.
Thanks in advance

Dear you can do it very easily with ajax. Ajax has data attribute which helps you pass your data from javascript to another page.
This link will help you a lot
https://api.jquery.com/jquery.ajax/

You can use session storage or cookies.
Example for session storage:
// First web page:
sessionStorage.setItem("myVariable", "myValue");
// Second web page:
var favoriteMovie = sessionStorage.getItem('myVariable');

You could use a query string to pass the value to the next page.
Add an ID to the form
<form class="form-horizontal" role="form" method="post" action="Page2.php" id="order-form">
Update the action of the form to add this query string from our JS variable
var id = Math.random();
$('#order-form').attr('action', 'Page2.php?id=' + id);
Get this variable in PHP (obviously you might wanna do more checks on it)
<? $id = $_GET['id'] ?>
We can now use $id anywhere in our PHP and we'll be using the ID generated from JS. Neat, right? What if we want it in JS again though? Simply add another script tag and echo it there!
<script type="text/javascript">
var id = <? echo $id ?>;
</script>
EDIT: Updated to add a little about how it works as you said you're not too sure about the transition between PHP and JS.
PHP runs on the server. It doesn't know much about the browser, and certainly doesn't know about JS. It runs everything and finishes executing before the web page is displayed. We can pass PHP variables to JS by creating script tags and creating a new javascript variable, echoing the PHP value.
JS (JavaScript) runs in the browser. It doesn't know about anything that happens on the server; all it knows about is the HTML file it is running in (hit CTRL+U to see raw HTML). As JS runs at a completely separate time to PHP there is no easy way to transfer variables (e.g. $phpVar = myJSVar). So, we have to use server methods like POST or GET.
We can create a GET or POST request in 2 main ways:
Using a form
Using an AJAX request
Forms work in the way I've outlined, or you can create a hidden field, set the value you want and then check for that. This involves redirecting to another page.
AJAX (Asynchronous Javascript And Xml) works slightly differently in that the user doesn't have to leave the page for the request to take place. I'll leave it to you to research how to actually program it (jQuery has a nice easy API for it!), but it basically works as a background request - an example would be displaying a loading spinner whilst loading order details from another page.
Hope this helps, let me know if something's not clear!

Including imagegrabscreen php function to a button onclick without refreshing the page

I'm trying to create a button with an onclick function that activates the imggrabscreen php function. Problem is, I've done several codes and so far the only function that I was able to use was a submit input type in which this refreshes the page. I tried using button as an input type but unfortunately, it does not save any screenshots upon clicking the button. Here's the code that I'm using so far.
if(isset($_POST['btnscreen']))
{
$im = imagegrabscreen();
imagepng($im, "screenshot.png");
}
ob_end_flush();
?>
<form method="post" action="" enctype="multipart/form-data">
<br>
<input type="submit" value="Click to Screenshot" id="btnscreen" name="btnscreen"></center>
<br><br>
</form>

php is parsed and executed server side (pre-processing) so you cannot call any php functions after the page has been sent to the browser. The only way to do this is to make a new request to the server (ajax).
I can't quite grasp what your function does (php cannot make a screenshot of what your browser is displaying as it has no information of how it has been rendered - please note different browsers may display the page differently).
I would try reading up on html5 canvas element which can achieve that (e.g. https://html2canvas.hertzen.com/).
Hope this helps

How to get information from Javascript to MySQL database using PHP?

(NEWBIE ALERT)
Hello! I am working on developing a database that stores information that people enter onto these online surveys. I don't have experience with Javascript, but I have worked with PHP and MySQL before. I am currently stuck on how to store the data to the database. Here are a few things about the code:
The person that created the online surveys had the online surveys written in Javascript (saved as HTML files)
Each survey is written in a separate file
Each survey has multiple data to be stored
Every time the user hits the next button, it goes to another page of the survey
I've worked on a project similar to this before, but my forms were only a page, so whenever the user clicks the "Submit" button, I had it go to another webpage written in a separate PHP file (kind of like a "results" page).
WHAT I DON'T UNDERSTAND/NEED HELP ON:
How do I make this so that when the user hits the "Next" button, it not only goes to the next page (what it's doing right now), but also sends the info to be stored in the database?
These surveys should be filled by people on their own computers so the surveys are written in JS (client-side). The storing part should be written in PHP (server-side) and MySQL, correct? Does this mean that I have to create a separate PHP file to create the code for transferring the data to the database or can it all be done in the same file? (I would think that I would need to create a separate file, one for each survey.)
Here's a general structure of how the HTML file:
<!DOCTYPE html>
<html>
<head><link rel="stylesheet" type="text/css" href="survey.css">
<script>
function Q2(){
document.getElementById("Q").innerHTML = "does something...<button type='button' onclick='Q3()'>Next</button>";
function Q3(){
document.getElementById("Q").innerHTML = "does something...<button type='button' onclick='Q4()'>Next</button>";
function Q4(){
document.getElementById("Q").innerHTML = "does something...<button type='button' onclick='Q5()'>Next</button>";
//keeps going until the last question
</script>
</head>
<body>
<h1>Meeting 1</h1>
<p id="Q">Some text...<br><input type="text" name="tweet" style='height: 50px;width: 500px;'><br><br>
<button type="button" onclick="Q2()">Next</button>
</p>
</body>
</html>
I've done a bit of research and looked at a few textbooks. I think AJAX may be something that I need to use? But I'm not too sure. If possible, could someone explain to me what I should be doing? I would like to not only be able to find a solution for this, but understand it as well.
Thank you in advance!!

For sending data to a PHP page using JavaScript, I'd recommend using the jQuery framework, where you can do it in as simple a code as this:
function Q2(){
var tweet = $("input[name='tweet']").val();
$.post("your_receiving_page.php", { data : tweet }, function(response){ //POST to PHP page where $_POST["data"] is the tweet variable
//deal with PHP output here
console.log(response);
if(response=="success"){
//javascript code to go to next page etc.
}
}
}
That way, you make a PHP file called "your_receiving_page.php" (or whatever) and handle the posted data like so:
<?php
$tweet = $_POST["data"];
//do stuff with $tweet, e.g. put it in a database
//...
//then end the code with "success", which is what you're looking for in the JavaScript as a successful callback
exit("success");

jQuery Auto-submit form inside of a <div> loads in new page

I'm developing a project of "prettifying" of a part of an existing web application. There is a need of putting the existing code: search criteria form in one div, and search results in another div (forming a kind of tabs, but that's another story).
Using jQuery I was able to manage that, but right now I am struggling with the results page, which by itself is yet another form that auto-submits to another file (using document.form.submit()), which is the final search results view. This auto-submit causes that the final view quits the destination div and loads as a new page (not new window).
So, the flow is like that:
First file, let's call it "criteria.html" loads the search criteria form (inside of a div) + another div (empty) destined to be filled with search results.:
<div id="criteria">... form with search criteria here...</div>
<div id="results"></div>
On submit, using jQuery's "hide()" method, I hide the first div (surrounding the search criteria form), and make Ajax call to the second file, let's call it "results.php":
<script>
$("#criteria").hide();
$.ajax({
...,
url: "results.php",
success: function(data){
$("#results").html(data);
},
...
});
</script>
results.php searches according to given criteria, and displays an "intermediary form" (which returns as a data result of the ajax query above) with a lot of hidden fields, and at the end executes:
<script>document.form.submit();</script>
which submits to another file, let's call it "resultsView.php"
This line causes that a result page shows outside the div "results", as a new page.
As there is a lot of logic in those files (more than 700 lines each), the idea of rewriting this monster just gives me creeps.
And now the question: is this a normal behavior (opening the result outside div)?
I tried removing the document.form.submit() code and everything works fine (well, without showing the results from "resultsView.php"). It's this line that causes the viewport to reset. I also tried with empty pages (to eliminate the possibility of the interaction with contents of the pages) - still the same result.
I hope there is not too much text and the problem is clearly stated. Every suggestion of how to fix this will be greatly appreciated.

If I understand your question correctly, you need to process the final submit using ajax instead of <script>document.form.submit();</script> so that you can handle the results on-page. Traditional form submits will always reload/open the action page. If you want to avoid that you'll have to control the form submit response via ajax and handle the results accordingly... like you are doing with the first submit.
The only alternative I can think of is to make div id="results" an iframe instead, so that it contains the subsequent form submit. Of course, that unleashes further restrictions that may cause other troubles.

I am not sure if I understood your question, but maybe u can do something like this.
This is my JQuery script: [I just wait for the submission search. When it happens, I use the $.Post method to call a function that should return the Html results (You can include somenthing to hide any field you want using JQuery or css)].
<script type="text/javascript" src="http://code.jquery.com/jquery-1.3.2.js"></script>
<script type="text/javascript" src="http://code.jquery.com/jquery-1.4.1.js"></script>
<script type="text/javascript">
$(document).ready(function() {
$("form#searchForm").submit(function() {
var theCity = $("select#chooseCity").val();
var theName = $("input#searchInput").val();
$.post("callProvideSearchResults.php", {theCity: theCity, theName: theName}, function(data) {
$("div#searchResults").html(data);
});
return false
});
});
</script>
This is my Body: {it consists of the choice of a city, the a form to provide the name of the person you are lookng for and the place to provide the results.
<body>
<FORM id="searchForm">
<h2>Select the city: </h2>
<select id="chooseCity">
<?php
$theCitiesOptionsHTML = "cityOptions.html";
require($thePathDataFiles.$theCitiesOptionsHTML); / A large list of cities
?>
</select>
<h2> What is the name of the person </h2>
<P> <INPUT id="searchInput" TYPE="TEXT" SIZE=50></P>
<P><INPUT TYPE="submit" VALUE="search"></P>
</FORM>
<div id="searchResults">
<!-- Here: Search results -->
</div>
</body>
// Function callProvideSearchResults.php // Just call the function that makes all the job and echo the $Html page
<?php
include "provideSearchResults.php";
$theName=$_POST['theName'];
$theCity=$_POST['theCity'];
echo provideSearchResults($theName, $theCity);
?>
// provideSearchResults.php // Database connection and search
<?php
function provideSearchResults($theName, $theCity) {
include "databaseConnection.php";
//database Queries
// Generate $theHtml using strings or ob_start, for instance
return $theHtml;
}
?>