What does everyone mean by: 'spl:_autoload() is default implementation of __autoload()'

I got confused with one thing about php autoloading stuff: the spl_autoload() function. In every answer I found that this function is default implementation of __autoload. Shouldn't PHP define default implementation of __autoload() in itself and then if I explicitly create __autoload() it will just override it?

If i don't explicitly define __autoload() function in my php file, will be there the default implementation? Is spl_autoload() some kind of internal function, if yes, why is it in php doc then?

(If it's not an internal function)In every spl_autoload() example there isn't any call to this function, only spl_autoload_register with no parameters, spl_autoload_extensions and so on. why so? What am I missing?

Quoting from: What is Autoloading; How do you use spl_autoload, __autoload and spl_autoload_register?

set_include_path(get_include_path().PATH_SEPARATOR.'path/to/my/directory/');
spl_autoload_extensions('.php, .inc');
spl_autoload_register();
Since spl_autoload is the default implementation of the __autoload() magic method, PHP will call spl_autoload when you try and instantiate a new class.

So if I won't call spl_autoload_register(), it won't register the default implementation? Does the spl_autoload look into extensions set by spl_autoload_extensions(); and then import all files with these extensions from include path? Repeating question mentioned earlier: is spl_autoload() internal function?

I know that __autoload() is deprecated and I should use spl_autoload_register(). I just want to make sure that I know how it all works.

Thanks.

Solution

Repeating question mentioned earlier: is spl_autoload() internal function?

That's just the "default value" for spl_autoload_register if you don't pass an parameter. You could also call this function standalone, if you want to. The behaviour of spl_autoload could be configured with spl_autoload_extensions and set_include_path.

Internally spl_autoload takes the full qualified class name(fqcn) as the path to look for a class implementation. (Maybe with string replacements for directory seperators). Afterwards, it searches in every element of the class_include_path after the given file.

spl_autoload_extensions(".php");
spl_autoload_register();
$foo = new \foo\Bar();
// now spl_autoload tries to load the file foo/Bar.php inside your class path.

If you need something more complicated, you have to create your own callback for the autoloader. I.e. something like this

spl_autoload_register(function($class) {
    $path = 'classes' . DIRECTORY_SEPERATOR;
    // dont care about case
    $class = strtolower($class);
    // replace _ with DIRECTORY_SEPERATOR
    $name = str_replace('_', DIRECTORY_SEPERATOR, $class);
    // don't care about windows/unix
    $name = str_replace('/', DIRECTORY_SEPERATOR, $name);
    $file = $path . $name . '.php';
    if (file_exists($file)) {
        include ($file);
    }
});

Note: The example above doesn't care about the value of spl_autoload_extensions or set_include_path.