alert("<?php echo "asdasda";?>");
// PHP is a Server Side Language, and JS is a Client Side Language, so the simple answer is
// you can not "run php in javascript". You CAN, however, use PHP to render an html
// page that includes js code on the server, including php variables, and deliver that page to the client where the javascript will be executed.
PHP Server-side EX:
<!doctype html>
<html lang="en" class="h-100">
<body>
<h1>Hello World</h1>
<script type="text/javascript">
var Javascript_variable = <?php echo $php_variable; ?>;
</script>
</body>
</html>
JS Client-side EX:
<script type="text/javascript">
var Javascript_variable = "some string from the above php variabe";
</script>