PHP letter changes

gistfile1.txt

<?php 

function LetterChanges($str) {  

  // code goes here
  $vowels = array('a','e','i','o','u');
  $letters = range('a','z');
  /* the for loop will continue its iterations
  as long as $i is less than the number of letters in $str */
  for ($i=0; $i<strlen($str); $i++) {
   /*if the value of $str is found in $letters 
   then the character order of its value will increase by one */
    if (in_array($str[$i], $letters)) {
      $str[$i] = $str[$i] == 'z' ? 'a' : chr( ord($str[$i])+1 ) ;
    /* if the value of $str can be found in $vowels, 
    then the value of $str will be capitalized */
    if(in_array($str[$i], $vowels)){
        $str[$i] = strtoupper($str[$i]);
    }
    }
  }
  /* when the entire length of $str has been iterated through, 
  it's final value is returned and the for loop terminates */
  return $str; 
         
}
   
// keep this function call here  
echo LetterChanges(fgets(fopen('php://stdin', 'r')));  

?>

Looks good. Clever use of range and in_array to decide whether to change each character.