- preg_match: documentation ( source)
- str_replace: documentation ( source)
<?php
$fiv = '<p id="copyright">jdhjhdfjksdfhjhfksdjh<div>sdhsajhda<div>sadgashdghasgdasdS<DAsadasdasjdsahdasuhdasD>ASDAS>daslkdasjdiasduiasudasyduasydasyduiasuiasydiausyduisaydiuy</p>';
preg_match('/<p id="copyright">(.*)<\/p>$/i', $fiv, $k);
if( $k[1] ){
echo str_replace($k[1], 'guilherme', $fiv);
}