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How can I save javascript variable value in a file using PHP or any other language?

I would like to save a JavaScript variable value in a file.
For example,
<script>
var ref = document.referrer;
</script>
I would like to save the variable ref value in a text file using PHP.
<?php
$file = fopen("ref.txt", "a+") or die("Unable to open file!");
fwrite($file, ref);
fclose($file);
?>
How can I do it?

You can do this by --
AJAX : Using AJAX you can easily pass the JS variable to PHP.
GET/POST : first you have to create a form with a text box. Then you have to set the value of that text box from your JS code like $('#id').val(ref). AND then you have to submit the form to a PHP page where you can get the value by simple calling $_GET['data'] or $_POST['data'].

How to place php variable in javascript object in different? [duplicate]

This question already has answers here:
How do I pass variables and data from PHP to JavaScript?
(19 answers)
Executing PHP code inside a .js file
(5 answers)
Closed 2 years ago.
I am trying to fetch data from php, store it as json and load it into my javascript code.
I stored my variable in php like this:
$data = array[];
$data = json_encode($raw_data);
Now I want to retrieve that data and use it in my javascript file.
I tried to use the following code in my js file:
var data ="<?php echo $data ?>";
[screenshot]: https://i.stack.imgur.com/pDfsf.png

If the data is only needed once on page load, you can use a script bloc inside the PHP code and then use a JS function to fetch it. But if data is being updated according to the user's interaction with the page, keep in mind that you can't send data from server side in PHP to the client side in JS without using AJAX.
I suggest you read about and use one of these 3 methods:
XHR
fetch
or axios

You are declaring the string to the variable data by using quotes.
Please remove the quotes and that would behave as per your expectations.
So your code should be replaced by the below lines.
var data =<?php echo $data ?>;

var data = <?php echo $data ?>;
or
var data = JSON.parse("<?php echo $data ?>");

The most obvious problem is your quote marks, which make the variable a string in JavaScript, rather than a complex object with a data structure. So any references to properties etc in the variable would not work. The quote marks should be removed so that the data is rendered as an object literal into the emitted JavaScript.
But there's another signficant issue: if this is being done in a .js file as you state, then the PHP interpreter is not running there, so the echo won't work (quotes or no quotes), because it isn't executed by PHP and turned into data. You'd just see the PHP code directly embedded in the JS code.
You'd have to echo the data into a script block in the .php file, then call the appropriate function in your .js file and pass the data as a parameter. This will work nicely if the data is only needed once.
e.g.
PHP file
<?php
$data = array[];
$data = json_encode($raw_data);
?>
<script src="someJsFile.js"></script>
<script>
var data = <?php echo $data ?>; //inject the data into the JS as an object literal
someFunc(data); //pass the data to a function in someJsFile.js
</script>
JS file:
function someFunc(data) {
//your code to process the data
}
If you need to keep it updated during the lifetime of the page, then you'll need AJAX to be able to request new data from the server, as suggested in one of the other answers.

Codeigniter insert id model using javascript

How can I get the value of the productid in the php code?
function getunitstock(){
var productid = document.getElementById('product').value
<?php $prodbyid = $this->pos_model->get_productbyid(+productid+)?>
document.getElementById('unitprice').value = <?php echo $prodbyid['price'];?>
}

You can't use javascript variable in php code. Php code run's on the serverside and javascript code runs in the client side. You can't ask the browser to run php code.
<?php $prodbyid = $this->pos_model->get_productbyid(+productid+)?>'
So, this line won't get value of productid.
The only way to do what you are looking to do is either:
do a redirect to a PHP script or do an AJAX call to a PHP script

Assign js variable in PHP variable

function show_selling_price(sp,co)
{
$r=sp;
var e="select * from tbl where id='$r'; ?>";
document.getElementById('selling_price_'+co).value=e;
}
//php assign value does not work why?
//Please correct above code with proper description

You can not set a PHP variable from JavaScript. For example this is not possible:
var js_var = 10;
$php_var = js_var;
However you can set a JS variable from PHP:
var js_var = <?php echo $php_var ?>;
PHP is executed on the server side and then the result shows up on the browser.
JavaScript is executed on the client side once the server returns the response.
If you really need to pass a JS var then use AJAX to submit vars to the server.

It is not possible to assign a javascript variable to a php variable like your example above.
Javascript is a client side language whilst PHP is a server side language.
What you can do is to echo out php variables to a javascript code before it is rendered at the visitors web browser like:
<script>
var text = "<?php echo $variable; ?>";
</script>
If you are setting a variable in Javascript first, and then want to pass it on to PHP, you either have to use ajax, or use a html-form to post the information to the server

How to get PHP string value in javascript?

Hi have looking on various questions but none of them seem to help me. I have a php variable in my php code and I am trying to access that in my javascript when I do. . .
var thing = "<?php echo($phpvariable); ?>";
then when I do
alert(thing);
It comes out to be "<?php echo($phpvariable); ?>" in the alert statement
What am I doing wrong?

Your PHP is obviously not being parsed. Are you in a .php file? If you're in a .js file, you'll need the server to parse those (or, more safely, put the PHP part somewhere in the DOM that the JS can access)
However, you're doing it wrong:
var thing = <?php echo json_encode($phpvariable); ?>;
Note: no quotes. json_encode will take care of that for you.

If this code is in a function in javascirpt that executes on click or at a specific event, then:
You are writing PHP Syntax in javascript, there is no way that you load the page then you run the php code. PHP code runs on the server side, so before any other HTML Javascript code executes
Else if you want to dynamically set the variable thing in javascript when the page is first loaded, then most probably you meant to write in the php file:
var thing = <?php echo '"'.$phpvariable.'"'; ?>;