Exploring PHP indirect references

The power and flexibility of an interpreted language is illustrated in PHP by the possibility of indirect references and other similar constructions. An indirect reference is illustrated with the following code:

$field = 'illustration';
$$field = 'a test string';
echo $illustration;

What will happen is that the phrase 'a test string' will be output. This example is artificial, but we will see how powerful this kind of thing is in the next section, when we consider how data objects can be loaded from the database. In fact, with our OO orientation, it will only rarely be necessary to use the double (or more) dollar signs for flexible references. In many contexts, such as referring to an object property or creating a new object, PHP will let us write a variable name where we might ordinarily write an actual name. So, we can refer to $object->$field when we have previously loaded $field with the name of one of the properties of $object. The same technique...