Description:
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I Opened This because #48129 is set to "won't fix". But this affects major frameworks i.e. (ZendFramework, FLOW3).It would be nice to tell spl_autoload() not to lowercase all filenames so that these framworks can benefit from its performance.

To stay backwards compatible this could be done via php.ini or a function call.
I would suggest spl_autoload_casesensitive in php.ini or a method spl_autoload_casesensitive(bool $flag);

Another more compatible option could be handling this inside of spl_autoload().
Check for unmodified case first (i.e. Zend/Db/Adapter/Abstract.php) and if this is not available try lowercased (i.e. zend/db/adapter/abstract.php)

This Solutions won't break BC!

Test script:
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<?php

namespace Test;
use Zend\Controller;

slp_autoload_casesensitive(true);
spl_autoload_extensions('.php');
spl_autoload_register();

// this should load from correct ZF path: Zend/Controller/Front.php;
$request = new Controller\Front();

Expected result:
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Class should be loaded from Zend/Controller/Front.php in include path

Actual result:
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Class is always loaded from zend/controller/front.php in include path