<?php
/**
 * 每隔3個字符,用逗號進行分隔
 * @param string $str
 * @return string
 */
function splitStrWithComma ($str)
{
  $arr = array();
  $len = strlen($str);
  for ($i = $len - 1; $i >= 0;) {
    $new_str = "";
    for ($j = $i; $j > $i - 3 && $j >= 0; $j --) {
      $new_str .= $str[$j];
    }
    $arr[] = $new_str;
    $i = $j;
  }
  $string = implode(',', $arr);
  // 翻轉字符串自己實現
  // $string = strrev($string);
  for ($i = 0, $j = strlen($string) - 1; $i <= $j; $i ++, $j --) {
    $tmp = $string[$i];
    $string[$i] = $string[$j];
    $string[$j] = $tmp;
  }
  return $string;
}
$str = "1234567890";
$new_str = splitStrWithComma($str);
echo $new_str . "\n";