How to include banner in the middle of the text?

Question

I am rebuilding an old project, written in PHP with Mysql. For practical purposes, consider it as a news portal.

In the database there are several articles, which I list on the site by simple query, which generates the strings $title $subtitle and $content.

I need to put an advertising banner in the middle of the $content text. I thought of breaking $content into lines to then put the advertising code block, but preferably after the first or second paragraph.

So what is the most recommended way to do this, in practice? Break a string into rows and identify paragraphs (preg_split() ?), inject what I need after the second and print the rest?

Updating

Note: Paragraphs of $content are separated by <br><br>, recorded in the database.

I got to the code below, based on the comment of Dvdsamm, but it doesn’t work properly, showing only the text without including <div>BANNER</div> after the second paragraph as expected.

$conteudo = ''.$x['conteudo'].'';

// adiciona banner
function addBanner($i)
{
static $conta = 0;
$retorno = $i[0];
if(++$conta == 2){
    $retorno .= "<div>BANNER</div>";
}
return $retorno;
}

$txt = preg_replace_callback('#(<br><br>)#', 'addBanner', $conteudo);
// adiciona banner

echo"$txt";