mysqli_stmt_get_result function is PHP 5.3 or greater only. It does not exist for your PHP 5.2.x version (which is not supported any longer btw.).
The alternative is to use
mysqli_stmt_bind_result with variable bindings.
In your concrete example this has even the benefit that you do not need to assign the array members to variables, because you can bind the variables directly.
mysqli_stmt_get_result function was introduced because someone thought this would stand in your way and getting an array would have been easier.