关于介词短语在句中成分的用法,语法根基牢的高手进!

关于介词短语在句中成分的用法,语法根基牢的高手进!
she has promised to buy me a book for my birthday this year.
请问for my birthday做什么成分呢?
she has promised to buy me a book as a birthday present.里的as a birthday present呢?