Php page with slash parameter

recently I have noticed that on some of my php webpages it is possible to add a slash (/) at the end of URL and write some text, which somehow modifies the page, e.g. assume address (just address example, not available online)

http://mypage.com/index.php

i typed something like

http://mypage.com/index.php/arbitrary_text

which causes the page to load with correct source code, but fails to load scripts, styles and images placed on same server (images whose source is outside the server load properly). I have seen this use of this type of address e.g. in CodeIgniter, which loads webpages using the parameters behind slash. However, i use no kind of this project and no htaccess file (and no php code), which would use and proccess such type of address, so the mystery for me is why do the parameters behind slash (in php page) load without styles, images and scritps, and somewhere the errors occur? Why does this happen? (I have tested in Chrome and Firefox, latest versions, Chrome shows broken images place holder, Firefox doesn't show any image) I have never seen this behaviour before. Could someone explain it (or navigate me through links, etc.)?!

For better imagination: this is a webpage with address of type mypage.com/gallery.php

Correct loading of a page

and this one with address of type mypage.com/gallery.php/xyz (just address example, not available online)

URL address with slash

Solution

As stated in comments, it is caused by PATH_INFO.