How to use PHP variable in a query

Hello, I’m having trouble using a variable in my graphql query. I think it’s clear what I’m trying to do here. This is in single.php

<?php

$post_slug=get_post_field( 'post_name', get_post() );

$graphql = graphql([
'query' => ' {
post(idType: SLUG, id: $post_slug) {
title
content
date
}
}'
]);

echo '<pre>';
var_dump($graphql);
echo '</pre>';
?>

The result says $post_slug is undefined.

array(2) {
  ["errors"]=>
  array(1) {
    [0]=>
    array(3) {
      ["message"]=>
      string(37) "Variable "$post_slug" is not defined."
      ["extensions"]=>
      array(1) {
        ["category"]=>
        string(7) "graphql"
      }
      ["locations"]=>
      array(2) {
        [0]=>
        array(2) {
          ["line"]=>
          int(2)
          ["column"]=>
          int(24)
        }
        [1]=>
        array(2) {
          ["line"]=>
          int(1)
          ["column"]=>
          int(2)
        }
      }
    }
  }
  ["extensions"]=>
  array(1) {
    ["debug"]=>
    array(0) {
    }
  }
}