$test = "
  Example
  Example Example Example
  Example
  Example Example
  Example Example Example
  Example Example
  Example
  Example Example Example Example
  ";

function ecallback() {
  $replace = array("Example1", "Example2", "Example3", "Example4", "Example5");
  return $replace[array_rand($replace)];
}

$out = preg_replace_callback('#Example#', 'ecallback', $test);

print ($out);

function ecallback() {
  static $i = 0;

  $replace = array("Example1", "Example2", "Example3", "Example4", "Example5");
  $size = sizeof($replace);

  $out = $replace[$i];
  $i++;

  if ($i == $size) {
    $i = 0;
  }

  return $out;
}