<?php
$string = "this is a test STRING BRB LOL"; // Your input string
$exclude_words = array("the","a","of","is"); // Exclude analyzing these words
$capital_words = array("brb","lol"); // Capital exclusives (leave in lowercase in array)
$max_length = 5; // Maximum word length, anything over is case-lowered

$parts = explode(" ",$string);
foreach ($parts as $word) {
 if (!in_array(strtolower($word),$exclude_words)) {
  $old_word = $word;
  $first_letter = substr($word,0,1);
  if (strlen($word) >= $max_length && !in_array(strtolower($word),$capital_words)) { $word = strtolower($word); }
  if (in_array(strtolower($word),$capital_words)) { $word = strtoupper($word); }
  $newWord = strtoupper($first_letter).substr($word,1);
  $string = str_replace($old_word,$newWord,$string);
 }
}

echo $string; // This should output "This is a Test String"
?>