这个spl_autoload_register是如何工作的

Question

I'm studying on spl_autoload_register(). I know what it is for ,just wondering how to use it practically. I searched for few references over the net for these, and each one has it's own style of approach. However I'm interested to understand the below script which found in one of my ebook.

WHat these three lines doing, I can understand roughly but not exactly. The autoload function receives a parameter $class, but why the below not dealing with it at all? For which class or item its defining the path?

1) get_include_path()

2) What is this doing? $flags = PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE;

         $paths = explode(PATH_SEPARATOR, get_include_path());
         $flags = PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE;
         $file = strtolower(str_replace("\\", DIRECTORY_SEPARATOR, trim($class, "\\"))).".php";

Script

function autoload($class)
{

     $paths = explode(PATH_SEPARATOR, get_include_path());
     $flags = PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE;
     $file = strtolower(str_replace("\\", DIRECTORY_SEPARATOR, trim($class, "\\"))).".php";

 foreach ($paths as $path)
 {
 $combined = $path.DIRECTORY_SEPARATOR.$file;
 if (file_exists($combined))
 {
 include($combined);
 return;
 }
 }
 throw new Exception("{$class} not found");
}
class Autoloader
{
 public static function autoload($class)
 {
 autoload($class);
 }
}
spl_autoload_register('autoload');
spl_autoload_register(array('autoloader', 'autoload'));
// these can only be called within a class context…
// spl_autoload_register(array($this, 'autoload'));
// spl_autoload_register(__CLASS__.'::load');

Answer 1

The function spl_autoload_register is called with parameter 'autoload'. That makes the function autoload to be called whenever a class is not found.

When a class is not found, then the function autoload is invoked with the faulty class name as its parameter, and that parameter is actually used in its third line.

$file = strtolower(str_replace("\\", DIRECTORY_SEPARATOR, trim($class, "\\"))).".php";

It is used to try to find a file with the same name as the class (Faulty class => Faulty.php file) in any of the directories returned by core function get_include_path.

The only odd thing is that the $flags variable was set to be included as one of the parameters for the str_replace function they are actually not used at all; but I don't know if they are really needed.