You can't define a constant that is another constant.  In the same versions
where define('FOO', TRUE) won't work, neither will define ('BAR',
__LINE__).  At least one of the DOS versions of php3 reliably demonstrates
this behaviour.  I'd call it a "no-no" to try and define a
constant based on another constant.

To get a full path (the equivalent of something like "__PATH__")
use 
dirname($SCRIPT_FILENAME)
to get the directory name of the called script and
dirname(__FILE__)
to get the directory name of the include file.

It may be useful to note that, in php4 (what version this started I don't
know, but it didn't do it before we upgraded to php4) __FILE__ will follow
symlinks to the origional file.

"SID" is defined also. It's the same as
"session_name()=session_id()" and usufull for managing session
using URL.
It's mentioned in Session reference, but I thought it would appropriate
mention this section, too.

"NULL" is also defined and has null type. gettype(null) returns
null and is_null($val) returns true for null - PHP4.0.4pl1 or later(?)

(I would like to know ALL constants defined by PHP in one place)

I also think that the class initilizer limitation intoruduced in PHP4
should be noted here. 

PHP4 does not allow;

define ('C1',1);
define ('C2',2);
class foo {
  var $val = C1 | C2;
}

Following code works.(PHP4.0.4pl1-linux)
define ('C1',1);
define ('C2',2);
class foo {
  var $val = C1;
  functin foo() {
    $this->val = C1 | C2;
  }
}

Details are explained 

(thanks to hholzgra)

If you use $PHP_SELF in forms or in places where a script has to reload
itself, you will get the full path relative to the web root. This is very
useful because it will always work, even if you move your script around on
the server. You don't need to mess around with absolute or relative paths.
An example would be:

<form method=POST
action=$PHP_SELF?do_something> or
<a
href=$PHP_SELF?do_something>Make script do something</a>

The pre-defined constant '__FILE__' does not work in same way at every
version of PHP.

Some version of PHP has the relative path, and some other has the absolute
path on __FILE__ constant..

Please be carefull in use..

[PS]
I have not tested at all versions of PHP but the version of 4.04pl.. and
4.05 are certainly not working in same way..  If you want to see that
bug(?), I can show you an example.

Constants have the same naming rules as variables (though the docs don't
actually say that). So you can't begin a constant name with a number.

if you name the
constant beginning with a numeral, such as:

define ("999CONSTANT", "some value");

whereas
define ("CONSTANT999", "some value");
 works fine.

Major difference between PHP3 and PHP4:
If you define a function that uses some constant C before you
define() C, C will be undefined in the function in PHP3 but not
in PHP4. Thus constants are more pre-processor-ish in PHP3,
but probably more useful in PHP4.

Also predefined (in PHP 4.1.1 on linux) are a set of LOG_* constants such
as LOG_DEBUG, LOG_ERR, LOG_WARNING, LOG_INFO, LOG_CRIT.  I hit this when
writing an external logging facility, and couldn't work out why some of my
constants had the wrong values e.g.

<?php
define("LOG_DEBUG", 10);
print LOG_DEBUG;
?>

results in printing "7" rather than the "10" I had
expected.  This cost me about 15 minutes of grief.  I would have expected
some warning about trying to redefine a predefined constant.

Warning, constants used within the heredoc syntax ()
are not interpreted!

Editor's Note: This is true. PHP has no way of recognizing the constant
from any other string of characters within the heredoc block.

The __FILE__ constant in 4.2rc1 (CLI) will return the location of script
specified to be run, rather than the absolute file.

eg. /usr/bin/phpmole (a softlink to /usr/lib/php/phpmole/phpmole.php)

started like this
bash#/usr/bin/phpmole 
 
the line echo __FILE__ in phpmole.php will output /usr/bin/phpmole - in
the CGI it would have returned /usr/lib/php/phpmole/phpmole.php

the workaround is to check for links!!
$f = __FILE__;
if (is_link($f)) $f = readlink($f);