Question

Does PHP read required scripts every time while processing new requests ?

Can you explain what disk IO operations are performed by PHP for processing single request

Does something change if PHP is an Apache module or PHP-fpm

Answer 1

Yes.

Imagine it this way:

script_a.php

<?php
$foo = 'bar';
$bar = 'foo';
?>

script_b.php

<?php
require_once 'script_a.php';
echo $foo . ' ' . $bar;
?>

At run-time, script_b.php will actually contain:

<?php
$foo = 'bar';
$bar = 'foo';
echo $foo . ' ' . $bar;
?>

So, it reads the script (or scripts) every time a new request is handled. This is why servers with medium to high load use opcode caches like APC or eAccelerator.

What these do is cache the whole script (with requires/includes processed) in memory so it doesn’t have to be processed to bytecode the next request but can just be executed. This can translate to a substantial performance increase because there is no disk I/O and the Zend engine doesn’t have to translate the script to bytecode again.

EDIT:

Does something change if PHP is an Apache module or PHP-fpm

No, at least not in the way PHP handles includes/requires.

Hope this helps.