Question Summary:
Is it permissible to wish someone well on their birthday?

Question Detail:

i do not celebrate my birthday nor in my family anyone does but my friends wish me on my birthday on phone or personally or on whats app so at such situation is it permissible to say just "thanks" to them . it will be rude if i don't reply . also can i wish them on their birthday since they feel bad if i don't .i do not indulge in their birthday parties. 

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.  
It is clear that you are aware that celebrating birthdays is not permissible. One requires lots of wisdom and diplomacy in correcting a wrong practice especially amongst close friends and associates.
In the enquired situation you may consider replying by saying “make dua” etc. There is nothing wrong in acknowledging a birthday of someone and making dua for the person. Celebrating birthdays as disbelievers do, is prohibited.[1]
And Allah Ta’āla Knows Best
Muhammad Haris Siddiqui
Student Darul Iftaa
Melbourne, Australia  
Checked and Approved by,
Mufti Ebrahim Desai.