Double letters:
th -> "ث"       gh -> "غ"
kh -> "خ"       sh -> "ش"
dh -> "ذ"

OR Non-Intuitive letters:
v -> "ث"        g -> "غ"
$ -> "ش"        x -> "خ"
* -> "ذ"

Different cases:
d -> "د"        t -> "ت"
D -> "ض"        T -> "ط"
z -> "ز"        h -> "ه"
Z -> "ظ"        H -> "ح"
s -> "س"
S -> "ص"

All other "normal" letters:
a -> "ا"        q -> "ق"   
b -> "ب"        k -> "ك"
j -> "ج"        l -> "ل"
H -> "ح"        m -> "م"
r -> "ر"        n -> "ن"
E -> "ع"        w -> "و"
f -> "ف"        y -> "ي"