How would i script this in Php?

[Chezzplay Chessplay]

First array has and index of a,b,c,d and the value of one,two,three,four.Second array is empty.I need to put the value of the first array to be the index of the second array and the first array index to be the value of the second array.I am new to this and trying to find the right function and code to run it but cant find one as of right now.
examples below;
$firstArray [a] = "One";
$firstArray = "Two";
$firstArray [c] ="Three";
$firstArray [d] ="Four";

$secondArray [] ="";
$secondArray [] ="";
$secondArray [] ="";
$secondArray [] ="";