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<?php

/**
 * Mutibyte string replace.
 * @param  string $string
 * @param  string $replace
 * @param  int    $position
 * @param  int    $length
 * @link http://php.net/manual/ja/ref.mbstring.php
 */
function mb_substr_replace($string, $replace, $position, $length) { 
  $str = mb_substr($string, 0, $position).$replace;
  return $str.mb_substr($string, (mb_strlen($str) + $length)); 
}

$string = '１２３４５６７';
$replaced = mb_substr_replace($string, '-', 3, -1);

var_dump($replaced);  // string(22) "１２３-４５６７"