<?php
 
$data =  "<p class=\"title\">Title one</p>";
$data .= "<p class=\"title\">Title two</p>";
$data .= "<p class=\"title\">Title three</p>";
 
$reg = 	"/<p.*?class=[\"']*title[\"']*.*?>.*?<\/p.*?>/si";
 
$repl = "<p class=\"my\">Replace on two</p>";
 
echo $data, " -------> ";
 
$repl_counter = 0;
 
echo preg_replace_callback($reg, function($match) use (&$repl_counter, $repl)
	{
	$res = $match[0];	// default
	if ($repl_counter == 1)	// second occurrence
		$res = $repl;
	$repl_counter++;
	return $res;
	}, $data);

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