Incorrect string value saving a string with emoji into db

Hi to all,

I have a web application for iphone where the user can write a complex text. When he uses an emoji icon and I try to store it into the db I get the error "incorrect string value".

Googling, I found that I have to to convert the mysql table charset to utf8mb4. I tried but I have the same problem.

Could you help me to understand how can I save the emoji icon?

Thank you!