How to Use Sessions and Session Variables in PHP

Session handing is a key concept in PHP that enables user information to be persisted across all the pages of a website or app. In this post, you’ll learn the basics of session handing in PHP. We’ll start with an explanation of how sessions work and how they are related to cookies. Then we’ll look at a few code snippets that demonstrate how to work with sessions. You’ll learn how to create and destroy sessions, and how to change session variables.

What is a Session in PHP?

A session is a mechanism to persist information across the different web pages to identify users as they navigate a site or app. Are you wondering why sessions are needed for a website? To see why sessions are necessary, we have to go back and see how the HTTP protocol is designed to work.

The HTTP protocol is a stateless protocol, which means that there’s no way a server can remember the specific user between multiple requests. For example, when you access a web page, the server is just responsible for providing contents of the requested page. So when you access other pages of the same website, the web server interprets each and every request separately, as if they were unrelated to one another. There’s no way for the server to know that each request originated from the same user.

The following diagram depicts the HTTP protocol in a nutshell.

In this model, if you wanted to display user-specific information, you’d have to authenticate a user in each request. Imagine if you had to type your username and password on every page that displayed your profile information! Yes, it would be cumbersome and not practical at all, and that’s where sessions comes into the picture.

A session allows you to share information across the different pages of a single site or app—thus it helps maintain state. This lets the server know that all requests originate from the same user, thus allowing the site to display user-specific information and preferences.

Login Flow With Sessions and Cookies

Let’s quickly go through a usual login flow for a website to understand what happens behind the scenes.

1. A user opens the login page of a website.
2. After submitting the login form, a server on the other end authenticates the request by validating the credentials that were entered.
3. If the credentials entered by the user are valid, the server creates a new session. The server generates a unique random number which is called a session id. It also creates a new file on the server which is used to store the session-specific information.
4. Next, a session id is passed back to the user, along with whatever resource was requested. Behind the scenes, this session id is sent in the PHPSESSID cookie in the response header.
5. When the browser receives the response from the server, it comes across the PHPSESSID cookie header. If cookies are allowed by the browser, it will save this PHPSESSID cookie which stores the session id passed by the server.
6. For subsequent requests, the PHPSESSID cookie is passed back to the server. When the server comes across the PHPSESSID cookie, it will try to initialize a session with that session id. It does so loading the session file which was created earlier during session initialization. It will then initialize the super-global array variable $_SESSION with the data stored in the session file.

In this way, the user data is preserved across the multiple requests and the user is kept logged in throughout a session.

The following diagram depicts how the HTTP protocol works with sessions.

Now that you’ve seen a brief introduction to how sessions work, we’ll create a few practical examples to demonstrate how to create and manipulate session variables.

How to Start a Session

In this section, we’ll discuss how to start a session in PHP.

Whenever you want to deal with session variables, you need to make sure that a session is already started. There are a couple of ways you can start a session in PHP.

Use the session_start Function

This is the method that you’l see most often, where a session is started by the session_start function.

The important thing is that the session_start function must be called in the beginning of the script before any output is sent to the browser. Otherwise, you’ll encounter the infamous Headers are already sent error.

Automatically Start a Session

If there’s a need to use sessions throughout your application, you can also opt-in to starting a session automatically without using the session_start function.

There’s a configuration option in the php.ini file which allows you to start a session automatically for every requests—session.auto_start. By default, it’s set to 0, and you can set it to 1 to enable the auto startup functionality.

session.auto_start = 1

On the other hand, if you don’t have access to the php.ini file, and you’re using the Apache web server, you could also set this variable using the .htaccess file.

php_value session.auto_start 1

If you add the above line in the .htaccess file, that should start a session automatically in your PHP application.

How to Get a Session Id

As we discussed earlier, the server creates a unique number for every new session. If you want to get a session id, you can use the session_id function as shown in the following snippet.

That should give you the current session id. The session_id function is interesting in that it can also take one argument—a session id. If you want to replace the system generated session id with your own, you can supply it to the first argument of the session_id function.

It’s important to note that the session_id function must be placed before the session_start call when you want to start a session with a custom session id.

How to Create Session Variables

In this section, we’ll explore how to initialize session variables in PHP.

As we discussed earlier, once a session is started, the $_SESSION super-global array is initialized with the corresponding session information. By default, it’s initialized with a blank array and you can store more information by using a key-value pair.

Let’s go through the following example script that demonstrates how to initialize session variables.

As you can see that, we’ve started a session in the beginning of the script using the session_start function. Following that, we’ve initialized a couple of session variables. Finally, we’ve accessed those variables using the $_SESSION super-global.

When you store the data in a session using the $_SESSION super-global, it’s eventually stored in a corresponding session file on a server which was created when the session was started. In this way, the session data is shared across the multiple requests.

As we discussed, the session information is shared across requests, and thus the session variables initialized on one page can be accessed from other pages as well, until the session expires. Generally, a session is expired when the browser is closed.

How to Modify and Delete Session Variables

You can modify or delete session variables created earlier in the application the same way as for regular PHP variables.

Let’s see how to modify the session variables.

In the above script, we’ve checked if the $_SESSION[‘count’] variable is set in the first place. If it’s not set, we’ll set it to 1, otherwise we’ll increment it by 1. So, you if refresh this page multiple times, you should see that the counter is incremented by one every time! 

On the other hand, if you would like to delete a session variable, you can use the unset function as shown in the following snippet.

Thus, you can no longer access the $_SESSION[‘logged_in_user_id’] variable as it’s deleted by the unset function. So that’s how you can alter the session information.

How to Destroy a Session

In this section, we’ll see how you could destroy a session. In the previous section, we discussed the unset function which is used if you want to delete specific session variables. On the other hand, if you want to delete all session related data at once, you can use the session_destroy function.

Let’s try to understand how it works using the following example.

The session_destroy function deletes everything that’s stored in the current session. Thus, you will see a blank $_SESSION variable from the subsequent requests as the session data that was stored on the disk was deleted by the session_destroy function.

Generally, you would use the session_destroy function when the user is being logged out.

Conclusion

In this article, we’ve explored the basics of session handling in PHP. It’s a key concept which allows you to persist information across web pages.

In the first half of the article, we discussed the basic concepts of session and later on we created a few PHP examples to demonstrate how you could create and destroy sessions as well as manipulate session variables.