all the inputs (data) comes in PHP from html webpages, for input , you need to use form in html document ,And in that form , you need an action and a method.
then in PHP you get all the inputs using super variable $_POST OR $_GET and save in normal variable and process it as normal
Do you mean forms? Or input like the user click a key on their keyboard? If it is from a form, you can definitely do it in php. Though to do client side interactions you need JavaScript. JavaScript can listen for key presses in the client browser.
why don't work "$line = trim(fgets(STDIN));" in sololearn codes? simply the function tell me $line is empty (null), don't allow write ... but in console it's works! what happen?
