What does this code mean?

What does this code mean? the following code cannot tell the number, where is it? it's messy.
$ Site_url = strtolower ('http ://'. $ _ SERVER ['http _ host']. substr ($ _ SERVER ['php _ SELF '], 0, strrpos ($ _ SERVER ['php _ SELF'], '/index. php ')). '/shop/index. php ');

Reply to discussion (solution)

$ _ SERVER ['http _ host'] is your HOST localhost
Substr string truncation
$ _ SERVER ['php _ SELF 'is equivalent to your file address/shop/index. PHP
Strrpos: find the position where the string appears in another string.

Finally, what site_url obtains is localhost/shop/index. php.

You can understand the meaning of a function.
Then, it is easy to understand the echo one by one. first, echo $ _ SERVER ['php _ SELF ']; exit (); to see what it is. if you don't understand it, output it first.