change of Variables in "for"

[wxdqz]

in my "for"-loop if have a variable that I want to change from a to b

I thought about the following but obviously this is wrong

$variable=a
for($i=0;$i<10;$i++)
{
if ($varibale=a)
{
$variable=b;
}
else{
$variable=a;
}
}

Thanks in advance

Chris