Warping Wurm is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Phasing (This phases in or out before you untap during each of your untap steps. While it's phased out, it's treated as though it doesn't exist.) At the beginning of your upkeep, this creature phases out unless you pay . Whenever this creature phases in, put a +1/+1 counter on it.