<?php
    $hostName='localhost';
    $userName='xxxxxxxxx';
    $userPass='xxxxxxxxx';
?>

<?php
    include 'db.php';
    echo $hostName;
?>

<?php
    $settings = 'my setting';
?>

include_once "settings.php"
...
   class MyClass {
       ....
        include_once "settings.php" //does nothing since file is already included
       //I can't reach $settings from here.
    }
...

include_once "settings.php"
...
   class MyClass {
       ....
        include "settings.php" //includes file again
       //I can reach $settings here
    }
...

$settings = 'my setting';
...
   class MyClass {
       ....
       //I can't reach $settings from here.
    }
...

$settings = 'my setting';
...
   class MyClass {
       ....
       $settings = 'my setting';
       //I can use $settings here obviously
    }
...