echo, print and is_callable()/function_exists()

Because both echo and print are language constructs and not actual functions, you can't seem to use either is_callable() or function_exists() on either to determine if they are valid 'function' calls. Is there any other way to determine to check to see if I can execute either or? I'm writing a version of array_walk_recursive() (it doesn't exist in the version of PHP we are using) and if I try to pass in either echo or print as the function, nothing ever happens because neither resolves as an executable function... Any ideas as to how I can get around this? thnx, Chris