Joey Santiago, the guitarist for the Boston-bred band the Pixies, has checked into rehab to treat alcohol and drug issues, the band announced Friday. In a statement, singer Black Francis, drummer David Lovering, and bassist Paz Lenchantin said the band, whose CD “Head Carrier’’ is due Sept. 30, still plans to tour the UK and Europe in November and December. “This is the best thing that Joey could have done, and we’re very proud that he’s taken this step,’’ said Lovering. “We ask all of our fans to support Joey . . . on this road to recovery.’’ Monday, Santiago posted the Modern Lovers’ song “I’m Straight,’’ saying “My status.’’