Contributing Advent 8: The magic __FILE__ constant

This issue came up in the #xdebug IRC channel on Freenode:

18:25 <user> no magic constants for PHP ?
18:27 <user> __FILE__ evaluates to "xdebug://debug-eval"

The user noticed this issue in the Eclipse expression evaluator. A similar issue is also described a few times on the internet:

• https://netbeans.org/bugzilla/show_bug.cgi?id=228981

• http://ru-php.livejournal.com/1557788.html

• http://stackoverflow.com/questions/4924676/how-can-i-get-the-php-magic-constant-file-work-with-eclipse-and-pdt

With the hint in comment 2 in the first URL, I answered the report in an answer on StackOverflow (the third URL). I am repeating this answer (with some modifications) here:

---

The output you get is not incorrect. __FILE__ is a special constant that gets evaluated at parser time. In fact, even though it looks like a constant, it is really not. When the PHP script gets compiled, it would really read something like this:

// test.php
<?php
    "test.php";
?>

even though the script source was:

// test.php
<?php
    __FILE__;
?>

This means that after parsing, there is no such "constant" __FILE__ at all, as it has already been replaced.

This means that if you evaluate __FILE__ in an IDE, through DBGp's eval command with eval -- __FILE__ it can not return you a filename that is represented through the __FILE__ "constant". Instead, it uses the filename for the current context which is xdebug eval or in later versions, xdebug://debug-eval.

In essence, it's the same as doing this:

php -r 'eval("__FILE__;");'

Which also outputs:

Command line code(1) : eval()'d code

Xdebug looks for this sort of output format, and changes it to xdebug://debug-eval so that it can actually debug into eval'ed code.

__FILE__ works as expected in "normal" PHP source code, as can be proven with this snippet:

<?php
$currentFilename = __FILE__;
?>

And after it has been assigned to a variable, you can evaluate $currentFilename in your IDE.