如何在PHP中解析常量

Is it possible to parse the contents of a constant in PHP?

For example,

define('WHO_AM_I', 'My name is $_SESSION['who_am_i'].'); // setup the constant string
echo eval(WHO_AM_I); // something like this -- but the eval() returns an error

Please note that I do not know the value of the _SESSION var until I actually use the constant later in the script stream.

Thanks.

AMENDED WITH REASON FOR WANTING TO DO THIS

I want to pull "hard coding" out of my script and give the user the ability to configure certain taxonomy in their site. So while I was doing this I also wanted to create a quasi-dynamic constant that I thought I might be able to parse later in the script.

If it can't be done...then it can't be done.

Don't shoot me for asking the question though.

A FINAL COMMENT TO AVOID ALL THIS CONFUSION

The purpose of my question has nothing to do with the eval() function. I am actually regretting having put it in there in the first place.

I put the eval() in the question simply to demonstrate to stackoverflow members that I did a bit if prep on my question rather than asking an open ended -- hey give me a solution without having offered any stab at it myself. So please disregard the eval().

All I want to know is can I somehow craft a define() in an way that makes the assigned value parse-able later in my script. That's it, that's all.

AMENDMENT C

I know I can do the following although I don't want to do it this way:

define('PARSE_ABLE_CONSTANT_PART_A', 'My name is ');
define('PARSE_ABLE_CONSTANT_PART_B', '.');

...later down the script road...

echo PARSE_ABLE_CONSTANT_PART_A . $_SESSION['who_am_i'] . PARSE_ABLE_CONSTANT_PART_B;

I just don't want to do it this way if I can make it slicker using an embedded var in the constant.

Constants by definition don't allow you to set it with dynamic content.

Here is a quote from the php manual:

As the name suggests, that value cannot change during the execution of the script

You can see more by going here

You might be thinking of magical constants

This seems really fishy, as other users have pointed out. You could do something like this if you wanted:

define('WHO_AM_I', 'echo \'My name is \'.$_SESSION[\'who_am_i\'];');
eval(WHO_AM_I);

This will always just echo the variable. You need to eval an expression afaik.

Just read your edit. I think you would be better suited with an .ini file, or maybe a static class with static properties. Makes it much more flexible, and you avoid the eval. You are talking user-generated content from what I can see - subjecting that to an eval call seems highly insecure.

A quick example of a static class you could use:

<?php
class myConstants{
  public static function _($key){
    switch($key){
      case "WHO_AM_I":
        return "My name is ".$_SESSION['who_am_i'];
        break;
      case "OTHER_CONSTANT":
        // does some other evaluation and returns a string
        break;
    }
    throw new Exception("Constant isn't defined");
  }
}
?>

Then you can just echo myConstants::_('WHO_AM_I');