Headers Already Sent

One of the most common errors in PHP is the "Headers already sent" error. The error is so common that many PHP developers encounter it at some point in their learning journey. Over time, as developers become more experienced, they typically develop habits and practices that help them avoid this error. But it remains a common stumbling block, especially when editing or refactoring existing code or working in a new codebase.

It looks something like this:

Warning: Cannot modify header information - headers already sent

The error typically arises in the following scenarios:

• Whitespace Issues: Unexpected whitespace (spaces, newlines) before the opening <?php tag or after the closing ?> tag in PHP files. Often, this whitespace is in included files or configuration files.
• Early Output: Echoing or printing content before using functions that modify headers, such as header(), session_start(), or setcookie().
• Error Messages: Displaying error messages or warnings before header-related functions can also trigger this issue.

Why does it matter?

Imagine you're receiving a formal letter. This letter comes in an envelope. The envelope has some basic info on it, like your address and the sender's return address. Inside the envelope is the actual letter with its message.

In the web world:

• The "envelope" is like the "headers" of an HTTP response.
• The "letter" inside is the actual content or "body" of the response (like a web page).

Now, let's say someone sent you the letter without putting it in the envelope. You'd be confused because you're expecting a standard format: envelope first, then the letter.

Similarly, web browsers expect web servers (like where PHP runs) to first send headers ("envelope") followed by the content or body ("letter"). This is a convention of the HTTP protocol (how web servers and browsers communicate).

PHP cares because it wants to follow this convention correctly. When you try to send content (like with an echo or print statement) before sending headers, PHP gets confused. It thinks, "Oh no! I've already started sending the letter, but I haven't prepared the envelope yet!" And so, it gives the "headers already sent" error.

Why is this convention important? Headers tell the browser important details like what type of content is coming (e.g., a webpage, an image, a video), when the content was last updated, or special instructions like redirecting to another page. They're also used to set or read cookies. So, if you're trying to start a session or set a cookie after sending content, it won't work because the "envelope" (headers) should have been prepared first. Lastly, headers can tell the browser to cache the content or how to decode compressed content. Sending headers correctly ensures efficient and smooth communication between the server and browser.

In summary, it's all about maintaining order. Just like you'd expect a letter to come in an envelope, browsers expect content to come with the appropriate headers. And PHP ensures this order is maintained, hence the error if you try to do things backward.

Replicating the Error

Let's first replicate the "headers already sent" error with PHP sessions. Create a file called session_error.php:

echo "Some content";
 
session_start();

When you run session_error.php in your web browser, PHP will generate a warning similar to this:

Warning: session_start(): Cannot start session when headers already sent in /path/to/session_error.php on line 4

This error occurs because the echo statement produces output before session_start(), which tries to send headers to initiate a session.

Fixing the Error

To fix the problem, simply move the session_start() to the beginning of the script before any output:

session_start();
 
echo "Some content";

Now when you run the modified session_error.php, there will be no error, as the session headers are sent before any content.

By ensuring session_start() is called before any output, you can easily avoid the "headers already sent" error related to sessions.

Key Takeaways

• The "headers already sent" error occurs when there's an attempt to modify or send HTTP headers after output has been sent to the browser.
• Browsers expect headers first, then content. Sending content before headers violates this expectation.
• Common causes are spaces or newlines before the <?php tag or after the ?> tag, using echo, print, or other output functions/statements before header operations.
• The error can prevent operations like: redirecting users with header('Location: ...');, starting sessions with session_start(), and setting cookies with setcookie().
• Being aware of this error and understanding its causes is crucial, especially when editing or refactoring existing code.