Output is not buffered by default with PHP and the contents of the script is sent to the browser in chunks as it is created. PHP’s output buffering functions buffer the output giving control over when content is sent, allowing it to be compressed and allowing a mixture of headers and content to be coded without getting those "Cannot modify header information – headers already sent" error messages.
Cannot modify header information – headers already sent
The above error message is displayed when some content has already been sent to the web browser and then a header is set, such as setting a cookie, doing a page redirect and so on. Headers must be set before sending any content which is the reason for the error.
While it is best practise to ensure all headers at set before sending any content, sometimes it can be simpler to just use output buffering which will eliminate the error message. When output buffering is enabled, all the headers will be sent but the content will only be sent either at the end of the script or when explicitly done by the script.
How to enable output buffering
To enable output buffering simply add the following to the start of the PHP script:
ob_start();
To enable gzip compression to make the page content smaller do this:
ob_start("ob_gzhandler");
Note that if the gzhandler is being used, then using any of the other buffering functions as shown in examples below probably won’t work as expected and may cause issues with the browser rendering the page.
Nothing further needs to be done in the script after making one of the two function calls above; when it gets to the end buffering is complete and the entire content is sent to the browser either as plain text, or gzip encoded with the appropriate headers.
Empty the output buffer
To empty the current contents of the output buffer do this:
ob_end_clean()
Get the contents of the output buffer
It is possible to get the contents of the output buffer as a string and do something with it, such as save it to a file, send it as an email, log it and so on. Some PHP functions echo the output rather than return it as a string and sometimes it can be useful to use output buffering to get this content.
The first example below will get the content and return it into the $content variable. The current buffer is unaffected by this function call:
$content = ob_get_contents();
The second example gets the content but empties the buffer, discarding the content from it:
$content = ob_get_clean();
Explicitly flush the buffer
The buffer can be explicitly flushed at any time using one of several functions.
The first example below flushes the buffer by sending the buffered content to the browser and the emptying the buffer. Any further output will continue to be buffered.
ob_flush();
The second example flushes the buffer by sending the content to the browser, emptying the buffer, and no longer buffering. Any further content is sent directly to the browser.
ob_end_flush();
The final example flushes the output to the browser in the same way as ob_end_flush() and also returns it as a string:
$content = ob_get_flush();
Further reading
Be sure to read the PHP manual for further output buffering examples and functions. The user comments on many of the function reference pages are very useful.