PHP does not have different primitive types for arrays and structs. It is all an array. The only difference is, that a list contains just ascending integers as keys, starting from zero, the rest is a struct.

<?php
if (array_keys($array) === range(0, count($array) - 1)) {
    // Is an array
} else {
   // Is a struct
}

In the Zend Framework this differentiation is needed in the XmlRpc-component. I’ve submitted a patch to make it more efficient.