What can I use instead of eval() ?

Question

I have a string that stores some variables that must be executed to produce a result, for example:

define('RUN_THIS', '\$something.",".$somethingElse');

Which is then eval()-uated:

$foo = eval("return ".RUN_THIS.";");

I understand that eval is unsafe if the string that gets evaluated is from user input. However, if for example I wanted to have everything run off Facebook's HipHop which doesn't support eval() I couldn't do this.

Apparently I can use call_user_func() - is this effectively the same result as eval()? How is deemed to be secure when eval() isn't, if that is indeed the case?

Edit: In response to the comments, I didn't originally make it clear what the goal is. The constant is defined in advance in order that later code, be it inside a class that has access to the configuration constants, or procedural code, can use it in order to evaluate the given string of variables. The variables that need to be evaluated can vary (completely different names, order, formatting) depending on the situation but it's run for the same purpose in the same way, which is why I currently have the string of variables set in a constant in this way. Technically, eval() is not unsafe as long as the config.php that defines the constants is controlled but that wasn't the point of the question.

Answer 1

I can't see any reason why you can't just use double-quote string building.

$foo = "\$something,$somethingElse";

Answer 2

Kendall seems to have a simple solution, but I'll try to answer your other question:

Apparently I can use call_user_func() - is this effectively the same result as eval()? How is deemed to be secure when eval() isn't, if that is indeed the case?

call_user_func is actually safer than eval because of the fact that call_user_func can only call one user function. eval on the other hand executes the string as PHP code itself. You can append '; (close the string and start a new "line" of code) at the end of the string and then add some more code, add a ;' (end the line of code and start another string so that there is no syntax error), thus allowing the constant RUN_THIS to contain lots of PHP code that the user can run on the server (including deleting all your important files and retrieving information for databases, etc. NEVER LET THIS HAPPEN.

call_user_func doesn't let his happen. When you run call_user_func_array($func, $args) the user can only run a restricted set of functions because: (a) the function has to be user defined (b) you can manipulate $func to ensure the user isn't able to run any function he/she wants either by checking that $func is in a list of "allowed functions" or by prefixing something like user_ to the function names and the $func variable itself (This way the user can run only functions beginning with user_.