Echoing a FALSE in PHP doesn't print anything to the screen. This is why your first example didn't work and it produced invalid JS when the user wasn't logged in.

logintemp= ;

A simple way to do this is to make sure you json_encode the server side data so the JS can access it. Since JSON is a subset of JS, it will properly escape any content and produce a valid expression that can be assigned to a variable.

var isLoggedIn = <?php echo json_encode(is_user_logged_in()) ?>;

if (isLoggedIn) {
  window.dataLayer = window.dataLayer || [];
  window.dataLayer.push({
    'userLoggedIn': '1'
  });
} 
   
else {
   window.dataLayer = window.dataLayer || [];
   window.dataLayer.push({
     'userLoggedIn': '0'
   });
}

And the above can be simplified to

var isLoggedIn = <?php echo json_encode(is_user_logged_in()) ?>;
window.dataLayer = window.dataLayer || [];      
window.dataLayer.push({
   userLoggedIn: isLoggedIn ? '1' : '0'
});
   

See Variable from html to javascript

Generating JavaScript from PHP

Dynamically generated code, as suggested by @Clarus Dignus, makes it much harder to read and maintain the code as you try to align code within both environments.

Having a single place where all the server side data drives the code makes it a lot easier to debug the code since the JavaScript code itself is not changing, just a value.

All you need is an endpoint that will return 1 if the user is logged in, otherwise 0.

let XHR = new XMLHttpRequest();
XHR.open( 'GET', '/yourEndPoint' );

XHR.onreadystatechange = function() {
   if (this.status == 200) {
       window.dataLayer = window.dataLayer || [];
       window.dataLayer.push({
           'userLoggedIn' : XHR.responseText
       });
       //Do something else here
   }
};

XHR.send();

@Clarus Dignus at the Wordpress-Org forum offered a nice solution via a plugin that makes sure the header code is executed in the header. He first asked me to execute this, to confirm everything is working:

<?php
if ( is_user_logged_in() ) {
    echo 'Welcome, registered user!';
} else {
    echo 'Welcome, visitor!';
}
?>

Then I used his solution to do the dataLayer push and works perfectly!:

<?php
if ( is_user_logged_in() ) {
?>
<script>
//Your JavaScript for when user is logged in
</script>
<?php
} else {
?>
<script>
//Your JavaScript for when user is logged out
</script>
<?php
}
?>

The plugin I used for the code insertion: Head, Footer and Post Injections plugin.