"is it possible in php to prevent “fatal error: call to undefined function”?" Code Answer

No. Fatal errors are fatal. Even if you were to write your own error handler or use the @ error suppression operator, E_FATAL errors will still cause the script to halt execution.

The only way to handle this is to use function_exists() (and possibly is_callable() for good measure) as in your example above.

It's always a better idea to code defensively around a potential (probable?) error than it is to just let the error happen and deal with it later anyway.

EDIT - php7 has changed this behavior, and undefined functions/methods are catchable exceptions.