functions.php

<?php
function the_content_ad($content){

  if(is_single()){	
		$switch = 1;$cont_ad = 0;$parrafo = 0;
	
		for($i=0;$i<strlen($content);$i++)
	
		{
	
			if(substr($content,$i,4)=="</p>"){
	
				$parrafo++;
	
			}
	
			if(substr($content,$i,6)=="</pre>"){
	
				$parrafo++;
	
			}
	
			if(substr($content,$i,13)=="</blockquote>"){
	
				$parrafo++;
	
			}
	
			if(substr($content,$i,8)=="</table>"){
	
				$parrafo++;
	
			}
	
			if(substr($content,$i,5)=="</ul>"){
	
				$parrafo++;
	
			}
	
		}
	
		if($parrafo <= 3){
	
			$put = (int)($parrafo/2);$ad=0;
	
		}else if($parrafo > 3 and $parrafo <= 8){
	
			$put = (int)($parrafo/2);$ad=1;
	
		}else if($parrafo > 8){
	
			$put = (int)($parrafo/3);$ad=2;
	
		}
	
		for($i=0;$i<strlen($content);$i++)
	
		{
	
			if(substr($content,$i,4)=="</p>" or substr($content,$i,6)=="</pre>" or substr($content,$i,13)=="</blockquote>" or substr($content,$i,8)=="</table>" or substr($content,$i,5)=="</ul>"){
	
				if($switch == $put and $cont_ad != $ad){
	
					
	
						if(substr($content,$i,4)=="</p>"){
							$tag = "</p>";
							$i +=3;
	
						}
	
						if(substr($content,$i,6)=="</pre>"){
	
							$i +=5;
							$tag = "</pre>";	
						}
	
						if(substr($content,$i,13)=="</blockquote>"){
	
							$i +=12;
							$tag = "</blockquote>";	
						}
	
						if(substr($content,$i,8)=="</table>"){
	
							$i +=7;
							$tag = "</table>";	
						}
	
						if(substr($content,$i,5)=="</ul>"){
	
							$i +=4;
							$tag = "</ul>";	
						}
	
					$text = $text.$tag ."<div id='ad_".($cont_ad+1)."' class='ad_box'></div>";
	
					$cont_ad += 1;
	
					$switch = 1;
	
				}else{
	
					$text = $text."".substr($content,$i,1);
	
					$switch += 1;
	
				}
	
			}else{
	
				$text = $text."".substr($content,$i,1);
	
			}
	
		}
	
		return $text;

	}else{

		return $content;

	}
}

add_filter('the_content','the_content_ad');