<?php

$str = "the sun is yellow when the <wrong> This is also red sun not yellow red sun is not yellow some words </wrong> and here is 
yellow sun outside <wrong> , now yellow sun inside </wrong>";

$origArr = array("sun");
$replaceArr = array("apple");
$str = str_replace($origArr,$replaceArr,$str);

$domElem = new DOMDocument();
$domElem->loadXML("<temp_tag>".$str."</temp_tag>");

$nodes = $domElem->getElementsByTagName('wrong');


$index = 0;
while($nodes->item($index)->nodeValue)
  {
    $nodes->item($index)->nodeValue = str_replace($replaceArr,$origArr,$nodes->item($index)->nodeValue);
    $index++;
  }

echo $domElem->saveHTML();

/* remove the temp tags ....*/

?>

<temp_tag>the apple is yellow when the <wrong> This is also red sun not yellow red sun is not yellow some words </wrong> and here is 
yellow apple outside <wrong> , now yellow sun inside </wrong></temp_tag>