Unable to horizontally center 'm' with GD2 - php

Question

My goal is to draw a horizontally centered m. I therefore calculate the width of the letter, substract that value from the total width and finally divide by 2. The result should be the distance from the left (or equally from the right).
However, the 'm' is always misplaced. I also noticed that some fonts may not trigger the problematic behavior. Note that my script correctly works for all other latin characters.
Arial:
Bitstream Vera Sans:
<?php
$totalWidth = 100;
$totalHeight = 100;
$font = 'Arial.ttf';
$img = imagecreatetruecolor($totalWidth, $totalHeight);
$red = imagecolorallocate($img, 255, 0, 0);
$fontSize = 100;
$bbox = imagettfbbox($fontSize, 0, $font, 'm');
$width = max($bbox[2], $bbox[4]) - max($bbox[0], $bbox[6]);
$centeredX = ($totalWidth - $width) / 2;
imagettftext($img, 100, 0, $centeredX, 100, $red, $font, 'm');
imagepng($img, 'testcase.png');
imagedestroy($img);

Answer 1

There is a small space left of each letter, and this is different each letter. Somebody on PHP.net wrote a solution for this: http://www.php.net/manual/en/function.imagettfbbox.php#97357
You need to adjust your code a little bit.
$totalWidth = 100;
$totalHeight = 100;
$font = 'Arial.ttf';
// change letter to see it with different letters
$letter = "m";
$img = imagecreatetruecolor($totalWidth, $totalHeight);
$red = imagecolorallocate($img, 255, 0, 0);
$fontSize = 100;
$bbox = calculateTextBox($fontSize, 0, $font, $letter);
$centeredX = (($totalWidth - $bbox['width']) / 2);
// here left coordinate is subtracted (+ negative value) from centeredX
imagettftext($img, 100, 0, $centeredX + $bbox['left'], 100, $red, $font, $letter);
header('Content-Type: image/png');
imagepng($img);
imagedestroy($img);