PHP Heroku - 调用未定义的函数mb_detect_encoding（）

I'm trying to deploy some php project to heroku. I use composer with required dependency are slim framework and simple_http_dom. Here content of my composer.json:

{
    "require": {
        "slim/slim": "2.4.3",
        "shark/simple_html_dom": "dev-master"
    }
}

when I run this app locally, it's working like a charm.

my app is succeed pushed to heroku. the problem occured when I try to access it from browser. it showing up nothing. here is some clue of the error i've got from heroku logs command.

2014-08-31T00:14:38.052441+00:00 app[web.1]: [Sun Aug 31 00:14:37.566291 2014] [proxy_fcgi:error] [pid 60:tid 140467698300672] [client 10.2.162.208:58783] AH01071: Got error 'PHP message: PHP Fatal error:  Call to undefined function mb_detect_encoding() in /app/vendor/shark/simple_html_dom/simple_html_dom.php on line 1234
', referer: http://myapp.herokuapp.com/index.php/scrap/all
2014-08-31T00:14:38.052437+00:00 app[web.1]: 10.2.162.208 - - [31/Aug/2014:00:14:37 +0000] "POST /index.php/scrap/all/do HTTP/1.1" 500 - "http://myapp.herokuapp.com/index.php/scrap/all" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/37.0.2062.94 Safari/537.36"
2014-08-31T00:14:38.052443+00:00 app[web.1]: [31-Aug-2014 00:14:37] WARNING: [pool www] child 58 said into stderr: "NOTICE: PHP message: PHP Fatal error:  Call to undefined function mb_detect_encoding() in /app/vendor/shark/simple_html_dom/simple_html_dom.php on line 1234"

it said:

Got error 'PHP message: PHP Fatal error: Call to undefined function mb_detect_encoding() in /app/vendor/shark/simple_html_dom/simple_html_dom.php on line 1234 ', referer: http://myapp.herokuapp.com/index.php/scrap/all

This is the first time I got this error. I have another project run on heroku using same dependencies (but without composer) and it's smoothly working.

what should I do to fix this problem ?

The problem is that the mbstring extension isn't enabled by default, see https://devcenter.heroku.com/articles/php-support#extensions

Just add ext-mbstring as a dependency to your composer.json so it looks like this:

{
    "require": {
        "ext-mbstring": "*",
        "slim/slim": "2.4.3",
        "shark/simple_html_dom": "dev-master"
    }
}

Heroku will then auto enable the extension when you git push.