$stack = debug_backtrace();
$firstFrame = $stack[count($stack) - 1];
$initialFile = $firstFrame['file'];

// ======= PATHINFO ====== //
$x = pathinfo($url);
$x['dirname']      🡺 https://example.com/subFolder
$x['basename']     🡺                               myfile.php?
$x['extension']    🡺                                      php?k=blaa#12345 // Unsecure! also, read my notice about hashtag parts    
$x['filename']     🡺                               myfile

// ======= PARSE_URL ====== //
$x = parse_url($url);
$x['scheme']       🡺 https
$x['host']         🡺         example.com
$x['path']         🡺                    /subFolder/myfile.php
$x['query']        🡺                                          k=blaa
$x['fragment']     🡺                                                 12345 // ! read my notice about hashtag parts

//=================================================== //
//========== self-defined SERVER variables ========== //
//=================================================== //
$_SERVER["DOCUMENT_ROOT"]  🡺 /home/user/public_html
$_SERVER["SERVER_ADDR"]    🡺 143.34.112.23
$_SERVER["SERVER_PORT"]    🡺 80(or 443 etc..)
$_SERVER["REQUEST_SCHEME"] 🡺 https                                         //similar: $_SERVER["SERVER_PROTOCOL"] 
$_SERVER['HTTP_HOST']      🡺         example.com (or with WWW)             //similar: $_SERVER["ERVER_NAME"]
$_SERVER["REQUEST_URI"]    🡺                       /subFolder/myfile.php?k=blaa
$_SERVER["QUERY_STRING"]   🡺                                             k=blaa
__FILE__                   🡺 /home/user/public_html/subFolder/myfile.php
__DIR__                    🡺 /home/user/public_html/subFolder              //same: dirname(__FILE__)
$_SERVER["REQUEST_URI"]    🡺                       /subFolder/myfile.php?k=blaa
parse_url($_SERVER["REQUEST_URI"], PHP_URL_PATH)🡺  /subFolder/myfile.php 
$_SERVER["PHP_SELF"]       🡺                       /subFolder/myfile.php

// ==================================================================//
//if "myfile.php" is included in "PARENTFILE.php" , and you visit  "PARENTFILE.PHP?abc":
$_SERVER["SCRIPT_FILENAME"]🡺 /home/user/public_html/parentfile.php
$_SERVER["PHP_SELF"]       🡺                       /parentfile.php
$_SERVER["REQUEST_URI"]    🡺                       /parentfile.php?abc
__FILE__                   🡺 /home/user/public_html/subFolder/myfile.php

// =================================================== //
// ================= handy variables ================= //
// =================================================== //
//If site uses HTTPS:
$HTTP_or_HTTPS = ((!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS']!=='off') || $_SERVER['SERVER_PORT']==443) ? 'https://':'http://' );            //in some cases, you need to add this condition too: if ('https'==$_SERVER['HTTP_X_FORWARDED_PROTO'])  ...

//To trim values to filename, i.e. 
basename($url)             🡺 myfile.php

//excellent solution to find origin
$debug_files = debug_backtrace();       
$caller_file = count($debug_files) ? $debug_files[count($debug_files) - 1]['file'] : __FILE__;

//(let's say, if wordpress is installed in subdirectory:  http://example.com/wpdir/)
home_url()                      🡺 http://example.com/wpdir/        //if is_ssl() is true, then it will be "https"
get_stylesheet_directory_uri()  🡺 http://example.com/wpdir/wp-content/themes/THEME_NAME  [same: get_bloginfo('template_url') ]
get_stylesheet_directory()      🡺 /home/user/public_html/wpdir/wp-content/themes/THEME_NAME
plugin_dir_url(__FILE__)        🡺 http://example.com/wpdir/wp-content/themes/PLUGIN_NAME
plugin_dir_path(__FILE__)       🡺 /home/user/public_html/wpdir/wp-content/plugins/PLUGIN_NAME/  

echo realpath(dirname(__FILE__));