Insert HTML between paragraphs in WordPress post content

functions.php

<?php
/**
 * Insert HTML between paragraphs in WordPress post content using php DOMDocument().
 *
 * Inserts HTML in the middle of top level paragraphs.
 * For example:
 *
 *     <p>Hello</p>
 *     <blockquote>
 *         <p>Awesome</p><!-- not a top level paragraph -->
 *     </blockquote>
 *     <!-- HTML is inserted here -->
 *     <p>World!</p>
 */


add_filter( 'the_content', 'insert_HTML_between_paragraphs' );

function insert_HTML_between_paragraphs( $content ) {

	/* Edit the HTML variables */
	
	// Block-level HTML element that's inserted between paragraphs (e.g. p, div, etc).
	$parent_element = 'p';

	// Content for the inserted block-level HTML element.
	$parent_content = "I'm <strong>inserted</strong> in the middle of top level paragraphs";

	/* That's all, stop editing variables here. */

	// HTML to be inserted.
	$insert_content = "<{$parent_element}>{$parent_content}</{$parent_element}>";

	// nodeDocument for post content
	$html = new DOMDocument();

	// Load the html from post content.
	@$html->loadHTML( $content, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD );

	// Get paragraphs from post content.
	$p = $html->getElementsByTagName( 'p' );

	// Get top level paragraph indexes
	$nodelist = array();
	foreach ( $p as $key => $node ) {
		if ( 'body' === $node->parentNode->nodeName ) {
			$nodelist[] = $key;
		}
	}

	// Check if more than 1 paragraph is found.
	if ( 1 < count( $nodelist ) ) {

		// Get position where to insert the HTML.
		$position = floor( count( $nodelist ) / 2 );

		// nodeDocument for insert HTML.
		$insert_html = new DOMDocument();

		// Load the html from insert content.
		@$insert_html->loadHTML( $insert_content, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD );

		// Parent bloc-level HTML element from insert content.
		$parent_node = $insert_html->getElementsByTagName( $parent_element )->item( 0 );

		// Insert the HTML in the middle of post content.
		$p->item( $nodelist[ $position ] )->parentNode->insertBefore( $html->importNode( $parent_node, true ), $p->item( $nodelist[ $position ] ) );

		// Convert HTML back to string.
		$content = $html->saveHTML();
	} else {
		// Insert HTML after content if none or only one paragraph is found.
		$content .= $insert_content;
	}

	return $content;
}