function countSentences($str){
	return preg_match_all('/[^\s](\.|\!|\?)(?!\w)/',$str,$match);
}

countSentences("This is a test.  Hey!"); // returns 2
countSentences("What do you want?"); // returns 1
// etc.

This will count sentences that end in . or ? or ! and have a space after them.

Count sentences in a string

regex, php, simple, function

PHP