Assignment of an array element counts as a read

[sirbrillig]

Discovered in #305, an assignment of one element of an array does not get recorded as a write to that variable.

$foo = &$bar;
$foo['key'] = 1; // This counts as a "read" of $foo when it should be a "write".

I think in most cases this ends up not being an issue since clearly all the tests are passing but it could be the source of subtle bugs.