Conditions about returning Chandra's Phoenix in one's hand
Opponent only refers to players, not creatures. Chandra's Phoenix will not trigger if you target a creature.
Comprehensive Rules reference:

102.2. In a two-player game, a player’s opponent is the other player.
102.3. In a multiplayer game between teams, a player’s teammates are the other players on his or her team, and the player’s opponents are all players not on his or her team.