title: php find a substring only if it's not part of another substring
body: <p>I know how to find substrings using strpos, but I want to return True only if the letter t appears in a string, but not if that t is followed by "he".  For example...
  <code>$str="The lion and dog are hungry"</code>
  The result would be <code>Does not contain t</code> because the only t in the string was part of the word "The". 
   <code>$str="Their bedroom is ugly"</code> should also return false because "Their" starts with T H E and there's not other t in the string.
  <code>$str="The cat and the dog are hungry"</code> would result in <code>Yes, this string contains a t</code> because there's a t in CAT.</p>