Send JavaScript variable to PHP variable

It depends on the way your page behaves. If you want this to happens asynchronously, you have to use AJAX. Try out "jQuery post()" on Google to find some tuts.

In other case, if this will happen when a user submits a form, you can send the variable in an hidden field or append ?variableName=someValue" to then end of the URL you are opening. :

http://www.somesite.com/send.php?variableName=someValue

or

http://www.somesite.com/send.php?variableName=someValue&anotherVariable=anotherValue

This way, from PHP you can access this value as:

$phpVariableName = $_POST["variableName"];

for forms using POST method or:

$phpVariableName = $_GET["variableName"];

for forms using GET method or the append to url method I've mentioned above (querystring).

PHP runs on the server and Javascript runs on the client, so you can't set a PHP variable to equal a Javascript variable without sending the value to the server. You can, however, set a Javascript variable to equal a PHP variable:

<script type="text/javascript">
  var foo = '<?php echo $foo ?>';
</script>

To send a Javascript value to PHP you'd need to use AJAX. With jQuery, it would look something like this (most basic example possible):

var variableToSend = 'foo';
$.post('file.php', {variable: variableToSend});

On your server, you would need to receive the variable sent in the post:

$variable = $_POST['variable'];

As Jordan already said you have to post back the javascript variable to your server before the server can handle the value. To do this you can either program a javascript function that submits a form - or you can use ajax / jquery. jQuery.post

Maybe the most easiest approach for you is something like this

function myJavascriptFunction() { 
  var javascriptVariable = "John";
  window.location.href = "myphpfile.php?name=" + javascriptVariable; 
}

On your myphpfile.php you can use $_GET['name'] after your javascript was executed.