A day before I got some cases,
How to styling the first paragraph or first image on my post different from others/next paragraph, and
How to insert an advertise i.e. adsens/banner automatic after first paragraph.

When writing a new post, be it from the visual or html tab, when you press enter, wordpress automatically opens a new paragraph and closes the already opened one.

So here the way I’ve found and used,
$paragraphBefore use for insert a div class style different outside/before first paragraph
$paragraphAfter use for insert some code or anything to displaying after the first paragraph

", $content);
for ($i = 0; $i 
//your first paragraph style
Your Code Here
//put your code i.e. here
"; } ?>

The code insert in your single.php from your template, find and replace the code below