PDFlib radial shading

gistfile1.php

PDF_save($this->pdf);
PDF_setcolor($this->pdf, "both", "rgb", 0.0, 0.0, 0.0, 0.0);
$sh = PDF_shading(
  this->pdf, 
  "radial",  $this->pdf->PAGE_WIDTH/2, $this->pdf->PAGE_HEIGHT/2,
  $this->pdf->PAGE_WIDTH/2, $this->pdf->PAGE_HEIGHT/2,
  1.0, 0.5, 0.5, 0.0,
  "r0 0 r1 500"
);
PDF_rect(this->pdf, 0, 0, $this->pdf->PAGE_WIDTH, $this->pdf->PAGE_HEIGHT);
PDF_clip(this->pdf);
PDF_shfill(this->pdf, $sh);
PDF_restore(this->pdf);