Here is a function that lets you write a string with your own "font tracking" level (the amount of pixels separating each character). It uses imagettfbbox to determine the width of each character, so it doesn't discriminate against the skinnier of characters. For this example, let $t = the amount of distance in pixels you want to separate each character from its neighbors.
<?php
function ImageTTFTextWithTracking($im, $size, $angle, $t, $x, $y, $color, $font, $text) {
$numchar = strlen($text);
for($i = 0; $i < $numchar; $i++) {
$char[$i] = substr($text, $i, 1);
imagettftext($im, $size, $angle, ($x + $w + ($i * $t)), $y, $color, $font, $char[$i]);
$width = imagettfbbox($size, $angle, $font, $char[$i]);
$w = $w + $width[2];
}
}
?>
Be aware that it currently does not work for angles other than the 0 default (I have no need for that).