PHPSESSID appears in the form arbitrarily

Question

I have a login form written in PHP, and every time I launch the browser and go to the corresponding page, the first field of the form is as follows:
<input type="hidden" name="PHPSESSID" value="session_id_code" />
If I close the window, but I do not restart the browser, this will not happen. Any idea what is happening and why?

Thanks!

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Form Code:

 <?php if (condition) { ?> <form method="post" action=""> <dl> <dt>Email:</dt> <dd><input type="text" name="useremail" /></dd> <dt>Password:</dt> <dd> <input type="password" name="userpass" /></dd> <dt class="dt-buttons"><input type="submit" name="button_login" value="Login" class="button" /></dt> </dl> <input type="hidden" name="formkey" id="formkey" value="224ca00155w2fcda8906e1e40af03a71" /> </form> <?php } ?> 

The form is simple HTML and is not dynamically generated.

EDIT2:
As I said, when I first open the page after launching the browser, this happens. If you refresh the page after this, the hidden field will not appear.
Is it possible to have something to do with SSL certificate? And if so, why do some pages / forms behave this way and some do not?

Answer 1

It looks like you have included trans_sid (transparent session identifier). Using trans_sid can be a security issue, especially if your site links to external content or allows the exchange of links - it allows you to skip the session ID of the user as part of the URL, which means that the session is very vulnerable to theft.