Use a Bounding Box to Center Text in PHP

<?php//from ww  w .  j av  a  2 s  .  c om
     header ("Content-type: image/png");
   
     $text = "Bounding Box!";
     $font_size = 15;
     $height = 500;
     $width = 300;
   
     $im = ImageCreate ($width, $height);
     $grey = ImageColorAllocate ($im, 230, 230, 230);
     $black = ImageColorAllocate ($im, 0, 0, 0);
   
     $text_bbox = ImageTTFBBox($font_size, 0, "ARIALBD.TTF", $text);
     $image_center = $width / 2;
     $text_x = $image_center - round(($text_bbox[4]/2));
   
     ImageTTFText($im, $font_size, 0, $text_x, 10, $black, "ARIALBD.TTF", $text);
     ImagePng ($im);
     ImageDestroy ($im);
?>