<?php
 
function pesquisarStrpos($palheiro, $agulha, $pesquisa) {
    $count = 0;
    $pos = -1;
    do {
        $pos = strpos($palheiro, $agulha, $pos + 1);
        $count++;
    } while ($pos !== false && $count < $pesquisa);
    return $pos;
}
 
$texto="<p>primeiro paragrafo</p><p>segundo paragrafo</p><p>terceiro paragrafo</p>";
 
$pos = (pesquisarStrpos($texto, '</p>', 2));
 
$textoInicio=substr($texto,0,$pos+4);
 
$textoFim=substr($texto,$pos+4,strlen($texto));
 
$result=$textoInicio."<div>...</div>".$textoFim;
 
echo $result;

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