How to insert custom code into the WordPress post content

functions.php

<?php // dont include this line

//Insert code after second paragraph of single post content.
add_filter( 'the_content', 'sj_insert_code' );
function sj_insert_code( $content ) {
	$code = '<div>Code to be inserted goes here</div>';
	if ( is_single() && ! is_admin() ) {
		return sj_insert_after_paragraph( $code, 2, $content );
	}
	return $content;
}
	
//Parent Function that makes the magic happen
function sj_insert_after_paragraph( $insertion, $paragraph_id, $content ) {
	$closing_p = '</p>';
	$paragraphs = explode( $closing_p, $content );
	foreach ($paragraphs as $index => $paragraph) {
		if ( trim( $paragraph ) ) {
			$paragraphs[$index] .= $closing_p;
		}

		if ( $paragraph_id == $index + 1 ) {
			$paragraphs[$index] .= $insertion;
		}
	}
	return implode( '', $paragraphs );
}