Parse error: syntax error, unexpected '}' in /var/www/test.php(36) : runtime-created function on line 1
Now if you're using the return from create_function in an anonymous context you might also get a message like the following.
Warning: preg_replace_callback() [function.preg-replace-callback]: Requires argument 2, '', to be a valid callback in /var/www/test.php on line 36
After getting a Parse Error in the beginning, a subsequent warning can be downright baffling since parse errors generally halt script execution. Apparently parse errors within anonymous, create_function context only halt execution within that context though.
Now when you look into that initial error and the create_function code portion thinking maybe you used a curly brace to end a function call instead of a closing parenthesis and don't see a curly brace anywhere "baffled" can't begin to explain the feeling you might have.
The reason for the initial error likely has to do with a missing semi-colon in the code argument of your create_function call. For instance if your code simply runs the arguments through a function like the following.
$str = preg_replace_callback('#[ACEGIKMOQSUWY]+#', create_function('$a', 'return strtolower($a)'), $str);
That seems perfectly fine, however it needs to have the semi-colon to end the function call as in the following.
$str = preg_replace_callback('#[ACEGIKMOQSUWY]+#', create_function('$a', 'return strtolower($a);'), $str);
This fact can be extremely confusing if you've used preg_replace with the e modifier where this ending semi-colon isn't required.