- preg_replace: documentation ( source)
<?php
$str = <<<"MIXED"
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus pulvinar lorem lectus, vel aliquam est blandit accumsan. Maecenas nisi eros, ornare a euismod rutrum, rhoncus a massa.
Hello world
<b>Goodbye world</b>
MIXED;
$str = preg_replace('/^(.*)$/m',"<p>$1</p>",$str);
echo $str;