PHP: Difference include/eval

Question

If the code is the same, there appears to be a difference between:

include 'external.php';

and

eval('?> . file_get_contents('external.php') . '<?php);

What is the difference? Does anybody know?

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I am actually pretty sure there is a difference. I just found out. I have got a large piece of ugly generated php in my database*. By creating my own streamWrapper I am able to include it directly from the database. However, I wanted to see how this compared to eval in terms of performance. So, I wrote a query that gets the php from the database and eval'd it as above. However, while the include works, the eval does not...

Answer 1

AFAIK you can't take advantage of php accelerators if you use eval().

Answer 2

I found the answer myself. Brace yourself, it's ugly:

include 'external.php';

is not equivalent to

eval('?> . file_get_contents('external.php') . '<?php');

because it is equivalent to:

eval('?> . file_get_contents('external.php') . '<?php;');

Note the semicolon after <?php yuck, it's ugly, but it makes the code work. And as it turns out that in my case, eval was about 10-20% faster...

Answer 3

If you are using a webserver on which you have installed an opcode cache, like APC, eval will not be the "best solution" : eval'd code is not store in the opcode cache, if I remember correctly (and another answer said the same thing, btw).

A solution you could use, at least if the code is not often changed, is get a mix of code stored in database and included code :

• when necessary, fetch the code from DB, and store it in a file on disk
• include that file
• as the code is now in a file, on disk, opcode cache will be able to cache it -- which is better for performances
• and you will not need to make a request to the DB each time you have to execute the code.

I've worked with software that uses this solution (the on-disk file being no more than a cache of the code stored in DB), and I worked not too bad -- way better that doing loads of DB requests of each page, anyway...

Some not so good things, as a consequence :

• you have to fetch the code from the DB to put it in the file "when necessary"
  • this could mean re-generating the temporary file once every hour, or deleting it when the entry in DB is modified ? Do you have a way to identify when this happens ?
• you also have to change your code, to use the temporary file, or re-generate it if necessary
  • if you have several places to modifiy, this could mean some work

BTW : would I dare saying something like "eval is evil" ?

Answer 4

This lets you include a file assuming file wrappers for includes is on in PHP:

function stringToTempFileName($str)
{
    if (version_compare(PHP_VERSION, '5.1.0', '>=') && strlen($str < (1024 * 512))) {
        $file = 'data://text/plain;base64,' . base64_encode($str);
    } else {
        $file = Utils::tempFileName();
        file_put_contents($file, $str);
    }
    return $file;
}

... Then include that 'file.' Yes, this will also disable opcode caches, but it makes this 'eval' the same as an include with respect to behavior.