Find all specified characters in input and Dave to file.

function search_and_save_to_file($input, $character){
  $arr = str_split($input);
  $data = "";
  foreach ($arr as $ch) {
    if ($ch == $character) {
      $data .= $ch;
    }
  }
  file_put_contents("./data.txt", $data);
}

search_and_save_to_file("aabbccddee", "a");