Cruel World Demo
When we collect information online, there is the chance that malicious users might try to hijack the normal operations of our pages.
There are many techniques but one is where an evil doer takes advantage of so-called "self-referential" PHP pages.
Said pages are designed such that when a user hits 'submit' the information provided is processed and the page itself is reloaded in the browser.
In such a page, if the evil doer types some JavaScript in the URL of said page, that 'script' can be run when the page is refreshed or reloaded.