output buffering not showing contents - php

Question

I am fairly new to PHP, even more so to the ob_ functions, so help me understand this, as the manual is somehow does not provide a very simple example or reference.
I am assuming that "output buffering" is what delays and holds php from sending headers until full content is sent, and that may be why the header() function does not issue an error if ob_start() is declared above. If so, my question is how do I "buffer" only some contents instead of just mentioning ob_start() at the top of my script, which is greatly slowing down my application?
Example.
<?php
namespace App\Controller;
class Home extends Controller{
public function showHomePage()
{
$students = $pdo->query('SELECT id FROM students');
$view->showContent($students); // includes content.php
}
}
//content.php
<p> showing stundent by id </p>
<?php
showContent()
{
if(!$students){
header('Location: /404');
}else{
//show students
}
}
}
Now you can see in the above example that, as soon as content.php is loaded, it will issue header already sent sent error (if $students evaluates to false/null ) so, to hide this error, I placed ob_start() inside my howHomePage method as seen here
public function showHomePage()
{
$students = $pdo->query('SELECT id FROM students');
ob_start();
$view->showContent($students); // includes content.php
}
Now, with the above approach, I get no header errors, but I would like to close that buffer as soon as the showContent() method is executed. In other means, I don't want the ob_start() to apply only for that following function. I tried to do something like this
public function showHomePage()
{
$students = $pdo->query('SELECT id FROM students');
ob_start();
$view->showContent($students); // includes content.php
on_end_flush();
}
but now, the contents showContent() are not being shown

Answer 1

<p> showing stundent by id </p>
<?php
showContent()
{
if(!$students){
header('Location: /404');
}else{
//show students
}
}
}
This is a terrible way to code. You've already got your output baked in, which, as you've noted, prevents you from changing the header(). This is a major driver behind MVC, which holds that you need to segment your code and separate your view(HTML) from your controller(PHP). In this case, you've put a function inline with your HTML.
There's a couple of ways to work around this without having to resort to output buffering
Do the check on $students earlier in the page (like when you get/build the data set) and issue the 404 there.
Move your HTML into a separate template file (maybe check out Smarty to help with that) and then do your drawing there.

Answer 2

I had the same issue and solved it by adding:
ob_implicit_flush(true);
to the beginning of the php file. This outputs everything right away and you can take your other flush commands out.
http://php.net/manual/en/function.ob-implicit-flush.php
ob_implicit_flush() will turn implicit flushing on or off. Implicit
flushing will result in a flush operation after every output call, so
that explicit calls to flush() will no longer be needed.