- substr: documentation ( source)
- var_dump: documentation ( source)
- strpos: documentation ( source)
<?php
$teste = "sfgsdfgsdfgsdgf</body>\n sdfasdfasdfdsf \n </html>";
$pos = strpos($teste, '</body>');
if ($pos !== false) {
$html = substr($teste, 0, $pos);
var_dump($html . 'TESTETESTE' . substr($teste, $pos));
}