Understanding the 'ob_start()' Function in PHP

The 'ob_start()' function in PHP stands for 'Output Buffering Start.' It is a significant PHP function in the realm of improving performance and achieving more control over HTTP headers.

When we call the 'ob_start()' function, it enables us to store output data into a buffer before sending it to the browser. This process is known as output buffering.

Let's delve into a simple example to understand how 'ob_start()' functions:

<?php   
// Starting output buffering
ob_start();

echo "Hello, World!";

// Clear (erase) the output buffer
ob_clean();

echo "Goodbye, World!";
?>

In the code above, when we execute this PHP script, the output will be "Goodbye, World!". The reason for this is that using ob_start() has initiated the output buffering, storing the "Hello, World!" into the buffer. Afterwards, ob_clean() has cleared the buffer content before the "Goodbye, World!" has been echoed out. Thus, only the latter output is sent to the browser.

The output buffering has performance advantages. The combined smaller echoed strings reduce the number of write operations. Buffering can also concatenate HTTP headers sent by your script.

Best Practices:

• Employ 'ob_start()' at the very beginning of your PHP script. This practice ensures all the output (including errors, if any) will be accumulated in the output buffer.
• Do remember to use 'ob_end_flush()' or 'ob_end_clean()' at the end of the script to send the output/empty the buffer respectively.
• Monitor the size of your buffer. Filling up a very large buffer might deteriorate your script performance.

Downsides:

• Some functions like 'header()' or 'setcookie()' must be called before any output is sent. Hence, if 'ob_start()' is omitted, these functions can only be called at the beginning of the script.

In conclusion, while 'ob_start()' is not a beginner-level PHP concept, understanding and using output buffering correctly can dramatically improve the efficiency and flexibility of your PHP scripts.