How to name a variable with the value of another variable?

I shall assume that you want a compact syntax to make this practical to use.
I shall choose cs, standing for compound symbol:

cs[x__] :=
  ToHeldExpression @ ToString @ Row @ {x} /.
    {_[s_Symbol] :> s, _ :> $Failed}

func_[a___, Unevaluated @ cs[x__], b___] ^:= 
  ToHeldExpression @ ToString @ Row @ {x} /.
    {_[s_Symbol] :> func[a, s, b], _ :> $Failed}

Test:

i = 10;
cs["d", i] = 30;

d10

30

Any expressions can be used so long as their evaluated forms concatenate to a valid Symbol name:

cs[Pi, d10, "x"] = 86;

Pi30x

86

Reassignment is possible:

cs[Pi, d10, "x"] = 99;

Pi30x

99

If an invalid Symbol name is produced by the concatenation $Failed is returned:

cs[5, "x"] = 30

$Failed

(Symbol names cannot start with numbers.)

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Recommended reading:

• Assigning values to a list of variable names

• Elegant manipulation of the variables list

• How to 'merge' a list like FromDigits, but with a mixture of numbers and symbols?

• How do you programatically load data into symbols? (especially Kuba's answer)

Despite the accepted excellent answer by Mr.Wizard I think it is worth to point out that the standard idiomatic approach to the problem in Mathematica is to use indexed variables:

In[1]:= i = 10;
d[i] = 30;
Definition[d]
d[10] = 10;
Definition[d]

Out[3]= d[10] = 30

Out[5]= d[10] = 10