## notice errors.. first Undefined index, then Undefined variable

Hello,

I get the error:
Notice: Undefined index: other

the line with this error has the code:
\$a = \$_POST['other'];

I tried to fix the code by doing this:
if (isset(\$_POST['other'])){
\$a = \$_POST['other'];
}

but this just gave me a new error:
Notice: Undefined variable: a

I tried to define the variable:
if (isset(\$_POST['other'])){
var \$a;
\$a = \$_POST['other'];
}

but then a fatal error occurs (unexpected T_VAR )

Can anyone suggest how I get rid of the 'undefined index' error properly (rather than just hiding it) so that new errors don't keep popping up?