Can the Mah Jong force out the Phoenix as part of a straight in Tichu?
Yes, the 7 is forced out in this case. The rule for the wish is that if a player has any legal play that involves playing the 7, then he must do so.

Note that if the player had 5,6,Ph,8,9; then he would NOT be forced to play that straight; because he doesn't actually have a 7.