Insert Content to Article Body in WordPress

There are many cases where developers need to inject content to an article’s body. Displaying ads is the most common.

Insert Content/Ads After nth Paragraph

add_filter( 'the_content', 'tltemplates_add_content' );
function tltemplates_add_content( $content ) {
    $additional_content = '<div>This line is added from code.</div>';
    if ( is_single() ) {
        $paragraph_position = 0; 
    	$content = tltemplates_add_content_after_paragraph( $additional_content, $paragraph_position, $content );      
    }

    return $content;
}
function tltemplates_add_content_after_paragraph( $additional_content, $paragraph_position, $content ) {
    $paragraphs = explode( '</p>', $content );
    foreach ($paragraphs as $key => $paragraph) {
        //add closing p back
        if ( trim( $paragraph ) != '' ) {
            $paragraphs[$key] .= '</p>';
        }
        //add additional content at the wanted position 
        if ( $paragraph_position == $key ) {
            $paragraphs[$key] .= $additional_content;
        }
    }

    return implode( '', $paragraphs );
}