What are the differences between using flush and ob_flush in PHP for output buffering?

When using output buffering in PHP, the `flush()` function sends the current output buffer to the client immediately, while `ob_flush()` flushes the output buffer but does not send it to the client. This means that `flush()` will send the output buffer to the client and clear it, while `ob_flush()` will only clear the output buffer. It is important to note that `flush()` may not work as expected in some server configurations due to server-side buffering.

// Using flush() to send the output buffer to the client immediately
ob_start();
echo "Hello, World!";
flush();
ob_end_flush();

// Using ob_flush() to flush the output buffer without sending it to the client
ob_start();
echo "Hello, World!";
ob_flush();
ob_end_clean();