I was looking half the day for a documentation about how to set the damn underline...
I found it out myself now.. happy to be able to share my new knowledge with you out there.
$decoration can be those:
1 - Text will be normal
2 - Underline
3 - Upperline
4 - Stroke-through
(PECL imagick 2.0.0)
ImagickDraw::setTextDecoration — Specifies a decoration
This function is currently not documented; only its argument list is available.
Specifies a decoration to be applied when annotating with text.
No value is returned.