Output buffering

Output buffering is a PHP mechanism that controls the output of a script. Imagine we write down echo 'test'; within our PHP script and do not see anything on screen. How is that possible? The answer is output buffering.

The following piece of code is a simple example of output buffering:

<?php

ob_start();
sleep(2);
echo 'Chunk#1' . PHP_EOL;
sleep(3);
ob_end_flush();

ob_start();
echo 'Chunk#2' . PHP_EOL;
sleep(5);
ob_end_clean();

ob_start();
echo 'Chunk#3' . PHP_EOL;
ob_end_flush();

ob_start();
sleep(5);
echo 'Chunk#4' . PHP_EOL;

//Chunk#1
//Chunk#3
//Chunk#4

When executed within the CLI environment, we will first see Chunk#1 come out after a few seconds, then a few seconds after, we will see Chunk#3 come out, and, finally, a few more seconds after, we will see Chunk#4 come out.  Chunk#2 would never be output. This is quite a concept, given that we are used to having the echo construct outputting stuff just after it is called.

There are several output buffering related...