If you use an anonymous function, it will return the object that are expected.

spl_autoload_register(function ($myclass){
    $keyclass = substr($myclass, 0, 1);

    switch ($keyclass) {
        case 'c':
            if (file_exists("class".DIRECTORY_SEPARATOR.$myclass.".php") === true)
                require_once ("class".DIRECTORY_SEPARATOR.$myclass.".php");
            break;
        case 'i':
            if (file_exists("interface".DIRECTORY_SEPARATOR.$myclass.".php") === true)
                require_once ("interface".DIRECTORY_SEPARATOR.$myclass.".php");
            break;
        case 'a':
            if (file_exists("abstract".DIRECTORY_SEPARATOR.$myclass.".php") === true)
                require_once ("abstract".DIRECTORY_SEPARATOR.$myclass.".php");
            break;
        default:
            if (file_exists($myclass.".php") === true)
                require_once ($myclass.".php");
    }

/******************************/

var_dump(spl_autoload_functions()) return:

array(1) {
  [0]=>
  object(Closure)#1 (1) {
    ["parameter"]=>
    array(1) {
      ["$myclass"]=>
      string(10) "<required>"
    }
  }
}

<?php
spl_autoload_register(function ($name) {
    echo "Want to load $name.\n";
   
});

spl_autoload_register(function($className) {
    var_dump($className);
});

function unregister($className) {
    var_dump($className.' i will be the first');
}

spl_autoload_register('unregister');

var_dump(spl_autoload_functions());

===================

array(3) {
  [0]=>
  object(Closure)#1 (1) {
    ["parameter"]=>
    array(1) {
      ["$name"]=>
      string(10) "<required>"
    }
  }
  [1]=>
  object(Closure)#2 (1) {
    ["parameter"]=>
    array(1) {
      ["$className"]=>
      string(10) "<required>"
    }
  }
  [2]=>
  string(10) "unregister"
}