How to automatically detect input values HTML php

Hello so I want when an input value is more than 1 character I automatically want to do something, I mean by automatically is that I want to do the thing without using form actions. Just automatically detect when the input characters gets to one or more, I want to use something similar like this:

<?php $input = $_GET(“input”);   
if(strlen($input) >= 1)
//Something
?>

But as you may know (Get) is uses for something else. and I know the way I Described my question is kinda weird, but however any help appreciated!

source https://stackoverflow.com/questions/73913284/how-to-automatically-detect-input-values-html-php