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.

I use this so I can break out of PHP with a lot of HTML but not render it. It saves me from storing it as a string which disables IDE color-coding.

<?php
ob_start();
?>
<div>
    <span>text</span>
    <a href="#">link</a>
</div>
<?php
$content = ob_get_clean();
?>

Instead of:

<?php
$content = '<div>
    <span>text</span>
    <a href="#">link</a>
</div>';
?>

The accepted answer here describes what ob_start() does - not why it is used (which was the question asked).

As stated elsewhere ob_start() creates a buffer which output is written to.

But nobody has mentioned that it is possible to stack multiple buffers within PHP. See ob_get_level().

As to the why....

1. Sending HTML to the browser in larger chunks gives a performance benefit from a reduced network overhead.

2. Passing the data out of PHP in larger chunks gives a performance and capacity benefit by reducing the number of context switches required

3. Passing larger chunks of data to mod_gzip/mod_deflate gives a performance benefit in that the compression can be more efficient.

4. buffering the output means that you can still manipulate the HTTP headers later in the code

5. explicitly flushing the buffer after outputting the [head]....[/head] can allow the browser to begin marshaling other resources for the page before HTML stream completes.

6. Capturing the output in a buffer means that it can redirected to other functions such as email, or copied to a file as a cached representation of the content