substr_count() counts the occurrence of a substring in an input string.

<?PHP
    $str = "<li><a href=\"http://https://convert.idontcarewhatyouthink.net/index.php\">Perschon</a></li>"    
	echo substr_count($str, "li"); //result is 2
?>

If you want to count a pattern of substrings and need to use regular expression, the preg_match_all() function can do that.

<?PHP
    $str = "<li><a href=\"http://https://convert.idontcarewhatyouthink.net/index.php\">Perschon</a></li>"    
	$cnt = preg_match_all("/\<[^\>]+\>/",$str, $mats);
	echo $cnt;  //result is 4
?>