PHP IF/Type Issue

Trying to get some clarification on this. Have a test form where you submit a letter and it gives a corresponding message.

if ($letter = "A") { $message = "First letter."; }
if ($letter = "B") { $message = "Second letter."; }
if ($letter = "C") { $message = "Third letter."; }

It keeps giving the last $message possible, in this case "Third letter." I know it has something to do with the operator/conditional statement and = sign, just not sure what to use properly. Thanks!