PHP读取js变量值

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>PHP读取js变量值</title>
</head>
<body>
    <script>
        var a = "JonyGuan";
    </script>
    
    <?php

    $b = 0;

    $b = "<script>document.writeln(a);</script>";

    echo 'a变量='.$b;

    ?>

</body>
</html>