preg_replace_tags.php

$str = '<p>teacher is reading</p>
<h1>parent</h1>
<h2>TEACHER</h2>
<address>teacher is ready</address>
<h3>PARENT</h3>
<strong>parent</strong>';
//for word in string too
//i for insensitive
$resp = preg_replace('/<(p|h1|h2|address)>(.*?)(teacher)(.*?)<\/(p|h1|h2|address)>/i','<$1>$2student$4</$5>',$str);

var_dump($resp);

/*
response:
string '<p>student is reading</p>
<h1>parent</h1>
<h2>student</h2>
<address>student is ready</address>
<h3>PARENT</h3>
<strong>parent</strong>' (length=139)
*/