The following code shows how the PHP session works. The function my_session_start() does almost the same thing as session_start().
<?php
error_reporting(E_ALL);
ini_set('display_errors', true);
ini_set('session.save_path', __DIR__);
my_session_start();
echo '<p>session id: '.my_session_id().'</p>';
echo '<code><pre>';
var_dump($_SESSION);
echo '</pre></code>';
$now = date('H:i:s');
if (isset($_SESSION['last_visit_time'])) {
 echo '<p>Last Visit Time: '.$_SESSION['last_visit_time'].'</p>';
}
echo '<p>Current Time: '.$now.'</p>';
$_SESSION['last_visit_time'] = $now;
function my_session_start() {
 global $phpsessid, $sessfile;
 
 if (!isset($_COOKIE['PHPSESSID']) || empty($_COOKIE['PHPSESSID'])) {
 $phpsessid = my_base32_encode(my_random_bytes(16));
 setcookie('PHPSESSID', $phpsessid, ini_get('session.cookie_lifetime'), ini_get('session.cookie_path'), ini_get('session.cookie_domain'), ini_get('session.cookie_secure'), ini_get('session.cookie_httponly'));
 } else {
 $phpsessid = substr(preg_replace('/[^a-z0-9]/', '', $_COOKIE['PHPSESSID']), 0, 26);
 }
 
 $sessfile = ini_get('session.save_path').'/sess_'.$phpsessid;
 if (is_file($sessfile)) {
 $_SESSION = unserialize(file_get_contents($sessfile));
 } else {
 $_SESSION = array();
 }
 register_shutdown_function('my_session_save');
}
function my_session_save() {
 global $sessfile;
 
 file_put_contents($sessfile, serialize($_SESSION));
}
function my_session_id() {
 global $phpsessid;
 return $phpsessid;
}
function my_random_bytes($length) {
 if (function_exists('random_bytes')) {
 return random_bytes($length);
 }
 $randomString = '';
 for ($i = 0; $i < $length; $i++) { 
 $randomString .= chr(rand(0, 255));
 } 
 return $randomString;
}
function my_base32_encode($input) {
 $BASE32_ALPHABET = 'abcdefghijklmnopqrstuvwxyz234567';
 $output = '';
 $v = 0;
 $vbits = 0;
 for ($i = 0, $j = strlen($input); $i < $j; $i++) {
 $v <<= 8;
 $v += ord($input[$i]);
 $vbits += 8;
 while ($vbits >= 5) {
 $vbits -= 5;
 $output .= $BASE32_ALPHABET[$v >> $vbits];
 $v &= ((1 << $vbits) - 1);
 }
 }
 if ($vbits > 0) {
 $v <<= (5 - $vbits);
 $output .= $BASE32_ALPHABET[$v];
 }
 return $output;
}