php - remove amount from price then insert new price

So i have a question. I have no idea how i need to insert a search in google for it so i'm going to ask it here.

I have like 50.00 euro. in mysql table it is showed as

 Id | Username | Balance
 1  | Kev      | 50.00
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Now when i buy something that is worth like lets say 5.00 euro it would be 50.00 - 5.00 = 45.00 euro left on balance

 Id | Username | Balance
 1  | Kev      | 45.00
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Now my question is. How do i do this with a simply get old balance then insert the new balance into the table? Also i'm using PDO

Sorry for my nooby question but i have no idea how to insert this into a search in google.

~Kev