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If Phantasmal Image somehow enters the battlefield at the same time as another creature, Phantasmal Image can't become a copy of that creature. You may choose only a creature that's already on the battlefield.
If the chosen creature is copying something else (for example, if the chosen creature is another Phantasmal Image), then Phantasmal Image enters the battlefield as whatever the chosen creature copied.
Any enters-the-battlefield abilities of the copied creature will trigger when Phantasmal Image enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.
Phantasmal Image copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below) and it gains the Illusion creature type and the triggered ability. It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.
If the chosen creature is a token, Phantasmal Image copies the original characteristics of that token as stated by the effect that created the token. Phantasmal Image is not a token in this case.
The Illusion creature type and the triggered ability that Phantasmal Image gains as part of its copy effect are both copiable values that other effects may copy.
If the chosen creature has in its mana cost, X is considered to be 0.
