Spain's World Cup-winning goalkeeper Iker Casillas said he "does not know what the future will be" as he was discharged from hospital after having a heart attack.The Real Madrid legend, 37, suffered "an acute myocardial infarction" during training with Porto on Wednesday.Speaking outside the Hospital CUF Porto he said he would rest for "for a couple of weeks or a couple of months"."The most important thing is to be here," he told reporters."A few days ago I went through a difficult situation, something that can happen at any moment of life, but fortunately I'm fine. I am grateful and feel that I am very lucky."Thank you to all who cared, for I felt very dear and cherished. I can only get out of this situation with a smile."Casillas praised the Porto medical team's quick actions which "allowed the effect of the infarction not to be so serious".He added: "I do not know what the future will be, but the most important thing is to be here, to be able to speak and transmit to everyone how I am and how I feel."Casillas made 725 appearances for Real during a 16-year career at the Bernabeu, helping them win three Champions Leagues and five La Liga titles.He was also part of the Spain squad as they won two successive European Championships, in 2008 and 2012, and the World Cup in 2010.Having joined Porto in 2015, he won the Portuguese title last season and holds the record for the most Champions League appearances with 177.