Initial Setup

$man = new Person("Fred",37);
$reference1 =& $man;
$woman = new Person("Wilma",35);
$reference2 =& $woman;

The first assignment copies the return value of the new Person() function into the value slot of $man.

Then the reference operator sets the $reference1 variable to refer to the same value slot. The slot that $reference1 previously referred to is dropped and the memory will be reclaimed by the garbage collector.

And we repeat for $woman and $reference2.

Note that neither $man nor $reference1 is the 'real' variable — both have 'equal rights' in the PHP language.

Accessing Fields

$man->name = "Barney";
$reference2->name = "Betty";

Because references have equal rights, it does not matter whether we use the reference variable or the original variable to access the record.

After assigning $man->name to "Barney", $reference1->name is also "Barney" — because $man and $reference1 refer to the same value slot, which holds the record formerly known as Fred.

Copying Values

$reference2 = $reference1;

// COPIES the value in
// $reference1 into the
// value in $reference2,
// which changes $woman

Here is where Java programmers need to pay attention!

The PHP assignment operator (=) copies the value slot of the first variable into the value slot of the second variable. It's exactly like copying one file onto another in the computer filesystem — so $woman->name is now "Barney" because the value slot referred to by $woman has been filled with data from $reference1.

Reassigning References

$reference2 =& $reference1;

// REASSIGNS $reference2 to
// refer to the same value
// slot that $reference1
// refers to

The reference operator (=&) does what the Java programmers are expecting. The variable $reference2 now refers to the same value slot that $reference1 refers to.

Unlinking References

unset($reference2);
$reference2 = "A String";

// After unsetting a variable
// you can safely assign it
// without accidentally
// clobbering other data.

So how do we avoid clobbering data when we do variable assignmetns? The unset() function breaks the connection between a variable and its value slot. The variable is now undefined, and when you make an an assignment a new value slot will be created and the given value copied in.

Note that if you were to unset $woman, the second value slot would no longer have any variables referring to it, and the garbage collector will reclaim the memory.

Value Slots Can Change Type!

$reference1 = 42;

// Note that when you copy
// a new value into a value
// slot, the new value can
// even be of a different
// type!  This can lead to
// hard-to-find bugs...

Just in case you missed the significance of "Copying Values" earlier, here is a more extreme example.

When we copy the number 42 into the value slot referred to by $reference1, that slot changes from holding a record to holding a number!

This means, among other things, than any subsequent references to $man->name will now crash your script, since $man no longer refers to a record and so the -> operator is illegal.