Check out these simple examples:

Example 1:
-----------------------------------------------------
<?php
function getCode()
{
    return("echo 'hello world!';");
};

eval(getCode());
?>
-----------------------------------------------------

 Result of executing whis example will be a string 'hello world!' as expected.

Example 2: 
-----------------------------------------------------
<?php
define('PHP_CODE','getCode()');

function getCode()
{
    return("echo 'hello world!';");
};

eval(PHP_CODE);
?>
-----------------------------------------------------

 As you can see - i just move code from eval() into constant
and substitute it with constant name instead of actual code into eval() statement.

 But result of executing this example is:

Parse error: parse error in test.php(9) : eval()'d code on line 1

 Tested with both 4.0.6 and 4.1.1 on windows 2000 and linux.