<?php

$text = "hello world this is a string";
$text = substr_replace($text, '', 10, 1);
echo $text . "\n";

$text = "hello world this is a string";
$text = str_replace(substr($text, 9, 1), substr($text, 7, 1), $text);
echo $text . "\n";

hello worl this is a string
heooo worod this is a string