An easier to manage thickness is, before to draw in the ellipse to play with 2 ellipse with different color :
<?php
imagefilledellipse ($this->image,60,42,57,57,$drawColor);
imagefilledellipse ($this->image,60,42,45,45,$backColor);
?>
The first line draw a filled ellipse with the wanted color, and the 2nd one, draw an ellipse with the background color from the same center, but is smaller.
The drawback of this method is that you erase everything in the middle of the ellipse.