Why Use Output Buffering in PHP

What is output buffering?

Output Buffering for Web Developers, a Beginner’s Guide:

Without output buffering (the default), your HTML is sent to the browser in pieces as PHP processes through your script. With output buffering, your HTML is stored in a variable and sent to the browser as one piece at the end of your script.

Advantages of output buffering for Web developers

• Turning on output buffering alone decreases the amount of time it takes to download and render our HTML because it's not being sent to the browser in pieces as PHP processes the HTML.
• All the fancy stuff we can do with PHP strings, we can now do with our whole HTML page as one variable.
• If you've ever encountered the message "Warning: Cannot modify header information - headers already sent by (output)" while setting cookies, you'll be happy to know that output buffering is your answer.

Output buffering vs. storing content into variable in PHP

The main differences:

1.) you can use "normal" output syntax, so for example an echo statement. You don't have to rewrite your problem.

2.) you have better control about the buffering, since buffers can be stacked. You don't have to know about naming conventions and the like, this makes implementations easier where the writing and using side are implemented separate from each other.

3.) no additional logic require to output buffered content, you just flush. Especially interesting if the output stream is something special. Why burden the controlling scope with dealing with that?

4.) you can use the same output implementation regardless of an output buffer has been created. THis is a question of transparency.

5.) you can 'catch' accidentially out bubbled stuff like warnings and the like and simply swallow it afterwards.

[...]

Is using output buffering considered a bad practice?

It's actually a good practice. Speed up data transfer

Shall I use output buffering (ob_start) or not

There are times where the use of output buffering is a good thing to use, but to use it as lots of people do (the lazy way of not having to send headers before output for example) is not the right time.

The example you give, I do not know much about, but if its optimal, it might be one of the times where its good to use.

Its not forbidden to use ob_start(), its just the "wrong way" to use it the way I stated earlier.

The optimization you mention feels like a very low-level optimization, you might get a tad bit 'quicker' output, but there is usually a lot of other optimizations in a standard php-script that could speed it up before that is worth looking at!

edit:
Example of a small script that don't use send headers before output vs one that do:

<?php
$doOutput = true;
$doRedirect = true;
$output = "";
if($doOutput == true){ 
    // $doOutput is true, so output is supposed to be printed.
    $output = "Some output yay!";
}
if($doRedirect == true){ 
    // but $doRedirect is also true, so redirect will be done.
    header("location:anotherpage.php");  // This will not produce an error cause there was no output!
    exit();
}
// The echo below will not be printed in the example, cause the $doRedirect var was true.
echo $output;

Instead of (this case will produce a header sent after output error):

<?php
$doOutput = true;
$doRedirect = true; 
if($doOutput == true){ 
    //Output will be printed, cause $doOutput is true.
    echo "Some output yay!";
}
if($doRedirect == true){ 
    // but $doRedirect is also true, so redirect will be done.
    header("location:anotherpage.php");  // This will produce an error cause output was already printed.
    exit();
}

edit2: Updated with a more obvious example!

What's the use of ob_start() in php?

Think of ob_start() as saying "Start remembering everything that would normally be outputted, but don't quite do anything with it yet."

For example:

ob_start();
echo("Hello there!"); //would normally get printed to the screen/output to browser
$output = ob_get_contents();
ob_end_clean();

There are two other functions you typically pair it with: ob_get_contents(), which basically gives you whatever has been "saved" to the buffer since it was turned on with ob_start(), and then ob_end_clean() or ob_flush(), which either stops saving things and discards whatever was saved, or stops saving and outputs it all at once, respectively.

Does output buffering in PHP require more resources?

I think it's best to use it with a high traffic site, or at least turn implicit flush off, to avoid sending partial responses over network, because it can slow down the rest of the script if the receiver is very slow too.

By sending the whole response in one time, you free all resources used by the php script, so it's more efficient.

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