Do all references to a string literal refer to same instance?

Question

One of my colleagues made a post that said something like this:

In PHP, if you have two variables referring to the same value, they are the same instance.

$a="Mary";
$b="Mary";
$c="lamb"

He implies that $a and $b refer to the same instance(memory space). I am having trouble beleiving this. I know that this is somewhat true in java, but I don't think its so for php, since in php strings aren't actually immutable by principle, it would not make sense to have one instance

Further,he said, if we do unset($a) it only removes the reference of $a not the actual value. This is ofcourse true, but proves nothing

I also tried the following code and printed both $a and $b. If they were sharing the same instance, the value of $b would have changed too.

$a[2]=3;
echo "<br/>\$a: $a<br/>"; //He3lo
echo "<br/>\$b: $b<br/>";//Hello

I would love to check the memory space of the variables, but I don't think php allows to do that. Can somebody clarify if this is true

Answer 1

You are referring to a concept called String interning. It seems that it is implemented in the Zend Engine since Version 5.4: "Our implementation makes the strings which are known at compile-time interned." Source.