To avoid the dreaded Headers Already Sent message we can turn on PHP’s output buffering:
# PHP
ini_sset('output_buffering', '1');
# .user.ini
output_buffering=On
# .htaccess
php_flag output_buffering On
The default value is off, so presumably it’s not always a good idea. I always try to write my PHP code so that it’s not necessarily.
However, what’s the disadvantage of turning output buffering on?
Output buffering is a mechanism in PHP that controls how output data (HTML, XML, JSON, etc.) from your scripts is sent to the client's browser. When output buffering is enabled, PHP will store the output data in a temporary buffer (think of it like a cache) instead of immediately sending it to the client. This buffer can be flushed manually or automatically, depending on your specific requirements.
This gives you the flexibility to capture and manipulate your scripts' output before sending it to the client, so you can do thinks like set headers after you've echo'd or outputted content in your script, which you normally can't do.
Output buffering can be useful for mitigating issues like "Headers Already Sent" (normally, headers must be sent before any actual output is sent), but it definitely come with trade-offs:
When you enable output buffering, you're instructing PHP to store the HTML output in memory until it's ready to be sent. For big pages with lots of content, this could lead to increased memory usage, potentially affecting the performance of your server or even leading to out-of-memory errors.
Due to how PHP manages output buffering, the client's browser won't actually start receiving data until PHP has fully processed the script, or until you explicitly flush the buffer. This could lead to a perception of slower page loads for end-users, especially for content-heavy pages.
Because the output is buffered, run-time errors (PHP errors/notices) aren't displayed immediately. This can make debugging more difficult as error messages won't appear in the place where they are triggered.
If you are working with real-time applications or streaming content, output buffering can be counter-productive because you would ideally want data to be sent to the client as soon as it is available.
Relying on output buffering to solve "Headers Already Sent" issues (depending on what's causing the issue) could be seen as a work-around, or a "hack job". If you're using output buffering to solve this problem, the better practice would be to structure your PHP code so that all headers are sent before any output, making the application more maintainable and easier to understand.
Here's a simple example demonstrating the benefits of output buffering:
// Without Output Buffering
header('Content-Type: text/plain');
echo "Hello, World!";
// Output buffering allows us to change headers even after sending output
ob_start();
echo "Hello, World!";
header('Content-Type: text/plain'); // This would not work without output buffering
ob_end_flush(); // Flush buffer, send output to browser