All kinds of ways in php..
$array = array('abcde');
$array[] = 'abcde';
Etc... not too sure what you're going for.
Edit: Oh, I think you might want to convert the first variable? Like this?
//Define the string
$myString = 'abcde';
//Convert the same variable to an array
$myString = array($myString);
Edit 2: Ahh, your comment above I think clears it up a little. You're getting back either an array or a string and don't know which. If you do what I just said above, you might get an array inside an array and you don't want that. So cast instead:
$someReturnValue = "a string";
$someReturnValue = (array)$someReturnValue;
print_r($someReturnValue);
//returns
Array
(
[0] => a string
)
$someReturnValue = array("a string inside an array");
$someReturnValue = (array)$someReturnValue;
print_r($someReturnValue);
//returns
Array
(
[0] => a string inside an array
)