/* 
@param: $len - number of sentences to return
@param: $content - string to be returned
*/
function limit_sentences($len, $content){
  $content = preg_split('/[\.\!\?]\s{1}/',$content);
  $o = null;
  for($i=0;$i<$len;$i++){
    $o.= $content[$i] . '. ';
  }
  return $o;
}

/* 
@param: $content - string to be returned
@param: $sentences - number of sentences to return
*/
function limit_sentences($content, $sentences) {

	$text = html_entity_decode(strip_tags($content));
	$text = explode('.', $text);
	for( $i=1; $i<=$sentences; $i++ ){
		$output .= $text[$i] . '. ';
	}
	if($output){
		return $output;	
	}
	else {
		return $content;
	}
}

A simple function that will split a given string into sentences, and then return the number of sentences specified. Quick and dirty.

String to sentences function

php

PHP