session_id

It may be good to note that PHP does not allow arbitrary session ids. The session id validation in PHP source is defined in ext/session/session.c in the function php_session_valid_key:
https://github.com/php/php-src/blob/master/ext/session/session.c
To put it short, a valid session id may consists of digits, letters A to Z (both upper and lower case), comma and dash. Described as a character class, it would be [-,a-zA-Z0-9]. A valid session id may have the length between 1 and 128 characters. To validate session ids, the easiest way to do it use a function like:
<?php
function session_valid_id($session_id)
{
 return preg_match('/^[-,a-zA-Z0-9]{1,128}$/', $session_id) > 0;
}
?>

session_id() itself will happily accept invalid session ids, but if you try to start a session using an invalid id, you will get the following error:
Warning: session_start(): The session id is too long or contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,'

You need to be careful if your page is expected to be accessed via GET requests from a smartphone. 
I noticed that when I explicitly (or via JavaScript) refresh a page on my smartphone using Chrome, the browser sends three simultaneous GET requests to get a complete page. In these requests, cookies are reset and the session ID is renewed, which might lead your PHP script to believe that these are new connexions.
I could not determine which criteria the browser uses to decide how many different requests should be sent. However, this does not happen with pages obtained from a POST request.
A word of advice: forms should not use the GET method if their processing involves writing data (database, file, email, etc.), but only if the subsequent actions simply involve reading data.