Here is how I added a dashed line to my pdf. I couldn't find any info on this, so I decided to write what I learned from trial and error.
On a page that is 612px wide and signified by $p, I drew a dashed line from left to right.
$p = pdf_new();
pdf_setdash($p,3,7); //3,7 can be any set of numbers
pdf_save($p); //This saves what already there
pdf_moveto($p,0,700); //move the pointer here
pdf_lineto($p,612,700); //draw a line
pdf_closepath_stroke($p); //show the line
pdf_restore($p); //put it all back
$buf = pdf_get_buffer($p);
$len = strlen($buf);
header("Content-Disposition: inline; filename=hello.pdf");