how to pass php variable to ajax?

The value from $_SESSION['loggedin'] needs to be transferred from the server PHP to the client by including it somewhere in the PHP generated HTML or PHP generated Javascript with
<?php echo $_SESSION['loggedin']; ?>

Within Javascript maybe

var ifloggedin = <?php echo $_SESSION['loggedin']; ?>;

OR

if(<?php echo $_SESSION['loggedin']; ?> == 1) {
window.location = "login-success.php";
}


Within HTML maybe

<script>
var ifloggedin = <?php echo $_SESSION['loggedin']; ?>;
</script>

I'd make sure to properly escape any user generated input before outputting it in a script context. XSS is really close by if you don't.

See rule 3: [owasp.org...]

Thanks for information.
Then seems for login form better not use ajax...
Better use only php for processing.