在PHP会话中无法处理计数器

Can you please take a look at this code and let me know why I am not able to increment the file numbers from 0 to ~ and just stocks on 0.html?

I am trying to use session counter and create files based on session counter from 0 to what ever number which page is visited

<?php
    session_start(); 
    $_SESSION["counter"] = 0;
    $file = fopen($_SESSION["counter"]++.'.html', 'w') or die("Unable to open file!");
    $tags = "<html>
    <head>
    <title>HTML email</title>
    </head>
    <body>
    <p>This is just something!</p>
    </body>
    </html>
    ";
    fwrite($file, $tags);
    fclose($file);
?>

You're setting the counter to zero every time someone visits the page!

Just do this:

if(!isset($_SESSION["counter"]))
    $_SESSION["counter"] = 0;