JS and PHP are two different animals and they do not mix together the way you are using those presently, which explains the syntax errors you are getting, because you are declaring what PHP thinks are constants.
What you need to do is to seperate those.
First, the JS/HTML:
<!DOCTYPE html>
<head>
<title></title>
</head>
<body>
<script type="text/javascript">
<!--
if (screen.width <= 800) {
window.location = "redirection_page.php?set=true";
}
else {
document.write ("Show them a message, or do something else.");
}
//-->
</script>
</body>
</html>
Then the PHP: (redirection_page.php)
<?php
if(isset($_GET['set']) && $_GET['set'] == true ){
header("Location: http://www.example.com/");
exit;
}
So, in your case that would read something like this:
N.B.: (You will need to do some modifications as to what the GET array will be).
<!DOCTYPE html>
<head>
<title></title>
</head>
<body>
<?php
$_GET['id'] = 2;
?>
<script type="text/javascript">
<!--
if (screen.width <= 800) {
window.location = "mplayer.php?id=<?php echo $_GET['id'] ?>";
}
else {
document.write ("Show them a message, or do something else.");
}
//-->
</script>
</body>
</html>
PHP: (mplayer.php)
<?php
if(isset($_GET['id']) && $_GET['id'] == 2 ){
echo $id = $_GET['id'];
}
Also make sure that you are not outputting before header (this means no HTML above the PHP, a space before the <?php tag, a cookie, etc. If this doesn't work for you, then your system may not be set to catch/display errors/notices/warnings, so enable error reporting.
If you do see a headers sent notice, read the following on Stack:
Add error reporting to the top of your file(s) which will help find errors.
<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);
// rest of your code
Sidenote: Displaying errors should only be done in staging, and never production.
