A complete example with namespaces:

fruits/pinapple.php

<?php
namespace Fruits;
echo "pinapple\n";
class Pinapple { }
?>

fruits/pinapple.php

<?php
namespace Vegetables;
use Fruits\Pinapple;
echo "carrot\n";
class Carrot { }
new Pinapple(); // Let's call autoload here
?>

index.php

<?php
spl_autoload_register(function($class_name) {
    @include_once(__DIR__ . '/' . strtolower(str_replace('\\', '/', $class_name)) . '.php');
});
new Vegetables\Carrot();
?>

Result:

carrot
pinapple

index2.php

<?php
spl_autoload_register(function($class_name) {
    @include_once(__DIR__ . '/' . strtolower(str_replace('\\', '/', $class_name)) . '.php');
});
spl_autoload_call('Fruits\\Pinapple'); // Reverse the load order
spl_autoload_call('Fruits\\Pinapple'); // Multiple call is safe with include_once
new Vegetables\Carrot();
?>

Result:

pinapple
carrot

It should be noted, that calling spl_autoload_call on a child class, and then on its parent class, throws a fatal error.

This happens because autoloading the child class also loads the class it extends. And since spl_autoload_call forcibly calls the registered autoload function(s), not taking into account whether the class exists, a fatal error is thrown:

File: child.class.php

<?php
class Child extends Parent () {
    public function __construct () {
        parent::__construct();
    }
}
?>

File: parent.class.php

<?php
class Parent () {
    public function __construct () {

    }
}
?>

File: autoload.php

<?php

/*    works fine    */
    spl_autoload_call('Child');

/*    throws: Fatal error: Cannot redeclare class Parent in /parent.class.php on line 2    */
    spl_autoload_call('Parent');

?>