performance of array cloning

Question

We all know that

$a1 = array('foo');
$a2 = $a1;
$a2[0] = 'bar';
// now $a1[0] is foo, and $a2[0] is bar. The array is copied

However, what I remember reading, but cannot confirm by Googling, is that the array is, internally, not copied until it is modified.

$a1 = array('foo');
$a2 = $a1; // <-- this should make a copy
// but $a1 and $a2 point to the same data internally
$a2[0] = 'bar';
// now $a1[0] is foo, and $a2[0] is bar. The array is really copied

I was wondering if this is true. If so, that would be good. It would increase performance when passing around a big array a lot, but only reading from it anyway (after creating it once).

Answer 1

It may be more than you wanted to know, but this article gives a good description of the way variables work in PHP.

In general, you're correct that variables aren't copied until it's absolutely necessary.