Only the JSON_ENCODE works for a variable from PHP to javascript

Only the JSON_ENCODE works for a variable from PHP to javascript

In the following, why echo 123 DOES NOT WORK?

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <script type="text/javascript">

$(document).ready(function () {
    var jsvar = <?php echo json_encode(123); ?>;     <---- //WORKS!!!

    var jsvar = <?php  echo 123; ?>;       // NOTHING HAPPENS!!! 

alert('ak=='+jsvar);      <---- //WORKS!!!
console.log(jsvar);       // NOTHING HAPPENS!!! 
 });//$document
</script>
<style>

  </style>
</head>
<body bgcolor="#E6E6E6">
</body>
</html>