php or unix command add watermark on pdf pages

how with php or unix command add watermark on (existing) pdf pages?

Now i use pdftk, but with this command i can't change waterma开发者_开发知识库rk position. I need watermark in bottom page.

You can still use pdftk to add the watermark/stamp. You should just create a PDF with your watermark/stamp in the correct position first, and then use either the background or stamp to combine it with your PDF.