PHP Accurate Font Width

I am trying to generate a text image.

So I create a new font:

   $font = '../fonts/Arial.ttf';

Then I add some background for the text

   imagefilledrectangle($image, 0, 0, ?, 12, $green);

And a开发者_StackOverflow社区dd text

   imagettftext($image, 12, 0, 0, 12, $white, $font, $text);

The problem is right now I don't know how to get the width of my font with size 12.

Thanks.

---

Using ImageTTFBBox()