What was the only thing that Philip wanted to take off the island with him?

In Chapter 19 of The Cay, the only thing Phillip wanted to take off the island with him was Timothy's knife.

During Phillip's rescue, one of the sailors went back to retrieve Timothy's knife that was stuck in the trunk of a palm tree. Both Phillip and Stew Cat were then escorted onto a destroyer which happened to be out hunting for German submarines.

When the ship docked at the naval base in Cristobal, Panama, Phillip was taken to the hospital. There, he was reunited with his parents. Phillip stated that he had to undergo three operations in order to restore his eyesight; even with the operations, Phillip knew that he would have to wear glasses for the rest of his life. After the war, Phillip and his family moved away from Scharloo and Curacao. However, Phillip reiterates that he will never forget Timothy, nor the Devil's Mouth. He thinks that, if he were ever to go back to visit Timothy's grave, he would recognize his cay just by closing his eyes.