Origin is at the lower left and the basic unit is the DTP pt.

1 pt = 1/72 inch = 0.35277777778 mm

Some common page sizes
Format Width Height
US-Letter 612 792
US-Legal 612 1008
US-Ledger 1224 792
11x17 792 1224
A0 2380 3368
A1 1684 2380
A2 1190 1684
A3 842 1190
A4 595 842
A5 421 595
A6 297 421
B5 501 709

Coordinate Example
<?php 
  $p = PDF_new(); 
  PDF_open_file($p); 
  PDF_set_info($p,"Creator","coords.php"); 
  PDF_set_info($p,"Author","Rasmus Lerdorf"); 
  PDF_set_info($p,"Title","Coordinate Test (PHP)"); 
  PDF_begin_page($p,595,842); 
  $font = PDF_findfont($p,"Helvetica-Bold","host",0); 
  PDF_setfont($p,$font,38.0); 
  PDF_show_xy($p, "Bottom Left", 10, 10);
  PDF_show_xy($p, "Bottom Right", 350, 10);
  PDF_show_xy($p, "Top Left", 10, 802);
  PDF_show_xy($p, "Top Right", 410, 802);
  PDF_show_xy($p, "Center",595/2-60,842/2-20);
  PDF_end_page($p); 
  PDF_set_parameter($p, "openaction", "fitpage");
  PDF_close($p); 
  $buf = PDF_get_buffer($p); 
  $len = strlen($buf);
  Header("Content-type:application/pdf");
  Header("Content-Length:$len"); 
  Header("Content-Disposition:inline; filename=coords.pdf");
  echo $buf; 
  PDF_delete($p); 
?>
Output