Description:
------------
spl_autoload_register() with no arguments registers spl_autoload(). But if we call spl_autoload_register() with a custom function before, it does nothing.
Nothing in the documentation seems to support this behaviour as valid.

Tested on PHP 5.5 Linux, 5.6 Win64 and 7.01 Win64.

Test script:
---------------
<?php
echo '<pre>';
/**
 * Every one of these code blocks must be run in separated.
 */
// Custom function first; spl_autoload after.
spl_autoload_register(function ($class) {});
spl_autoload_register();
print_r(spl_autoload_functions());	// Prints only one entry.
/*

// Custom function first; spl_autoload after.
spl_autoload_register(function ($class) {});
spl_autoload_register('spl_autoload');
print_r(spl_autoload_functions());	// Prints two entries, as expected.

// spl_autoload first; custom function after.
spl_autoload_register();
spl_autoload_register(function ($class) {});
print_r(spl_autoload_functions());	// Prints two entries, as expected.
*/

Expected result:
----------------
All blocks should print two entries.

Actual result:
--------------
All blocks should print two entries, except the first that only prints one.