A transparent background with text doesn't seem to work very well because of the antialiasing . But I tried the following kluge and it turned out very well:

<?php
   $im = imagecreatetruecolor(100,20);
   $almostblack = imagecolorallocate($im,254,254,254);
   imagefill($im,0,0,$almostblack);
   $black = imagecolorallocate($im,0,0,0);
   imagecolortransparent($im,$almostblack);
   //... set x and y..
   imagettftext($im,8,0,$x,$y,$black,"calibri.ttf",$txt);
?>