Validation: is_callable validation issue

[Maxximus]

In run_rule: is_callable with true as second parameter will only evaluate the first parameter as a possible function name (e.g. check if it is a string, or a valid array).

This does NOT mean the function will exist. so 'blah' will validate as true, but will give a PHP error if it doesn't exist.

Besides that, according to user comments (@ php.net) the results differ depending on PHP version.

[armen]

Fixed r1270