insert ad in content

gistfile1.php

function mobile_single_post_ad( $content ) {

	if( wp_is_mobile() && is_single() ) {
		$limit   = 100; //word limit
		$contents = explode("</p>", $content);
		$words = 0;
		$displayed = false;
		foreach ($contents as $paragraph) {
			$words = $words + str_word_count($paragraph);
			echo $paragraph . "</p>";
			if( $words >= $limit && $displayed == false ){
				$displayed = true;
				echo '<div id="mobile-post-ad">'; tdr_ad_widget( 'mobile' ); echo '</div>';
			}
		}
	} else { return $content; }
}
add_action('the_content', 'mobile_single_post_ad');