What does __FILE__ mean?

Question

I have the following code from Codeigniter index.php

My understanding is that,

If / of string position in $system_folder (in this case CIcore_1_7_1) is false, and if realpath function exists AND (?) is not false, $system_folder is assigned to (?) /$system_folder. else $system_folder is assigned to $system_folder with replacing \\ with /.

Q1. What does realpath function means?

Q2. What does this mean?

@realpath(dirname(__FILE__))

Q3. Am I right? Do I have any misunderstanding?

Q4. What kind of situation do you need the following?

str_replace("\\", "/", $system_folder)

---

$system_folder = "CIcore_1_7_1";

/*
|---------------------------------------------------------------
| SET THE SERVER PATH
|---------------------------------------------------------------
|
| Let's attempt to determine the full-server path to the "system"
| folder in order to reduce the possibility of path problems.
| Note: We only attempt this if the user hasn't specified a 
| full server path.
|
*/

if (strpos($system_folder, '/') === FALSE)
{
    if (function_exists('realpath') AND @realpath(dirname(__FILE__)) !== FALSE)
    {
        $system_folder = realpath(dirname(__FILE__)).'/'.$system_folder;
    }
}
else
{
    // Swap directory separators to Unix style for consistency
    $system_folder = str_replace("\\", "/", $system_folder); 
}

Answer 1

1. The realpath() function gives you the file-system path, with any symbolic links and directory traversing (e.g. ../../) resolved. The dirname() function gives you just the directory, not the file within it.

2. __FILE__ is a magic constant that gives you the filesystem path to the current .php file (the one that __FILE__ is in, not the one it's included by if it's an include.

3. Sounds about right.

4. This is to translate from Windows style (\) paths to Unix style (/).