How do I pass variables and data from PHP to JavaScript?

Sitebee

Technical SEO
Staff member
There are actually several approaches to do this. Some require more overhead than others, and some are considered better than others.

In no particular order:
  1. Use AJAX to get the data you need from the server.
  2. Echo the data into the page somewhere, and use JavaScript to get the information from the DOM.
  3. Echo the data directly to JavaScript.

In this post, we'll examine each of the above methods, and see the pros and cons of each, as well as how to implement them.


1. Use AJAX to get the data you need from the server​


This method is considered the best because your server-side and client-side scripts are completely separate.


Pros​

  • Better separation between layers - If tomorrow you stop using PHP, and want to move to a servlet, a REST API, or some other service, you don't have to change much of the JavaScript code.
  • More readable - JavaScript is JavaScript, PHP is PHP. Without mixing the two, you get more readable code in both languages.
  • Allows for asynchronous data transfer - Getting the information from PHP might be time/resources expensive. Sometimes you just don't want to wait for the information, load the page, and have the information reach whenever.
  • Data is not directly found on the markup - This means that your markup is kept clean of any additional data, and only JavaScript sees it.

Cons​

  • Latency - AJAX creates an HTTP request, and HTTP requests are carried over the network and have network latencies.
  • State - Data fetched via a separate HTTP request won't include any information from the HTTP request that fetched the HTML document. You may need this information (e.g., if the HTML document is generated in response to a form submission) and, if you do, will have to transfer it across somehow. If you have ruled out embedding the data in the page (which you have if you are using this technique) then that limits you to cookies/sessions which may be subject to race conditions.

Implementation Example​


With AJAX, you need two pages, one is where PHP generates the output, and the second is where JavaScript gets that output:

get-data.php
Code:
/* Do some operation here, like talk to the database, the file-session
* The world beyond, limbo, the city of shimmers, and Canada.
*
* AJAX generally uses strings, but you can output JSON, HTML and XML as well.
* It all depends on the Content-type header that you send with your AJAX
* request. */

echo json_encode(42); // In the end, you need to echo the result.
                      // All data should be json_encode()d.

                      // You can json_encode() any value in PHP, arrays, strings,
                      //even objects.

index.php
Code:
<!-- snip -->
<script>
    function reqListener () {
console.log(this.responseText);
}

var oReq = new XMLHttpRequest(); // New request object
oReq.onload = function() {
// This is where you handle what to do with the response.
// The actual data is found on this.responseText
alert(this.responseText); // Will alert: 42
};
oReq.open("get", "get-data.php", true);
// ^ Don't block the rest of the execution.
// Don't wait until the request finishes to
// continue.
oReq.send();
</script>
<!-- snip -->

The above combination of the two files will alert 42 when the file finishes loading.


Some more reading material​


2. Echo the data into the page somewhere, and use JavaScript to get the information from the DOM​


This method is less preferable to AJAX, but it still has its advantages. It's still relatively separated between PHP and JavaScript in the sense that there is no PHP directly in JavaScript.


Pros​

  • Fast - DOM operations are often quick, and you can store and access a lot of data relatively quickly.

Cons​

  • Potentially Unsemantic Markup - Usually, what happens is that you use some sort of <input type=hidden> to store the information, because it's easier to get the information out of inputNode.value, but doing so means that you have a meaningless element in your HTML. HTML has the <meta> element for data about the document, and HTML 5 introduces data-* attributes for data specifically for reading with JavaScript that can be associated with particular elements.
  • Dirties up the Source - Data that PHP generates is outputted directly to the HTML source, meaning that you get a bigger and less focused HTML source.
  • Harder to get structured data - Structured data will have to be valid HTML, otherwise you'll have to escape and convert strings yourself.
  • Tightly couples PHP to your data logic - Because PHP is used in the presentation, you can't separate the two cleanly.

Implementation Example​


With this, the idea is to create some sort of element which will not be displayed to the user, but is visible to JavaScript.

index.php
Code:
<!-- snip -->
<div id="dom-target" style="display: none;">
    <?php
        $output = "42"; // Again, do some operation, get the output.
echo htmlspecialchars($output); /* You have to escape because the result
will not be valid HTML otherwise. */
?>
</div>
<script>
var div = document.getElementById("dom-target");
var myData = div.textContent;
</script>
<!-- snip -->

3. Echo the data directly to JavaScript​


This is probably the easiest to understand.


Pros​

  • Very easily implemented - It takes very little to implement this, and understand.
  • Does not dirty source - Variables are outputted directly to JavaScript, so the DOM is not affected.

Cons​

  • Tightly couples PHP to your data logic - Because PHP is used in presentation, you can't separate the two cleanly.

Implementation Example​


Implementation is relatively straightforward:

Code:
<!-- snip -->
<script>
var data = <?php echo json_encode("42", JSON_HEX_TAG); ?>; // Don't forget the extra semicolon!
</script>
<!-- snip -->

The above was originally posted on Stack Overflow back in 2014. There are probably cleaner and more up-to-date methods now. The guide to passing variables and data from PHP to JavaScript did work for me. The reason I have copied it across here to this forum is for easy access only. Here is the original post: https://stackoverflow.com/questions/23740548/how-do-i-pass-variables-and-data-from-php-to-javascript
 
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