running R from PHP

Below are 2 scripts (png.R and pdf.R) I am trying to run in R from
PHP.  

     # png.R
     x11(display=":5")
     png(filename="g.png")
     plot(1:5)
     graphics.off()

     # pdf.R
     pdf(file="g.pdf")
     plot(1:5)
     graphics.off()

Both run from terminals and png.R will run without a normal X server
if Xvfb is running.  Neither runs under PHP, though (when invoked as
"R --no-save < xxx.R").  They yield the following errors (with the R
startup banner deleted for compactness):

     > x11(display=":5")
     > png(filename="g.png")

     Error in X11(paste("png::", filename, sep = ""), width, height, pointsize,  : 
	     unable to start device PNG
     In addition: Warning message: 
     could not open PNG file `g.png'
     Execution halted

and

     > pdf(file="/usr/pkg/share/httpd/htdocs/test-R/g.pdf")

     Error in PDF(file, old$family, old$encoding, old$bg, old$fg, width, height,  :
	     unable to start device pdf
     In addition: Warning message:
     cannot open `pdf' file argument `/usr/pkg/share/httpd/htdocs/test-R/g.pdf'
     Execution halted

For reference, the R banner information includes the following:

    R : Copyright 2003, The R Development Core Team
    Version 1.6.2  (2003-01-10)

Any help on how to get these scripts to work is greatly appreciated.