Remove a TODO that no longer applies in php 8.0

[TysonAndre]

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Any expression can be used as of 8.0 without an unexpected syntax error.

In encapsulated strings, "${a->prop}" is equivalent to "{${(a->prop)}}" in php 8.0 and a syntax error in previous php versions.

(Get the property prop of the global constant a (constants can be objects in php 8.1), then get the variable with the name equal to that string)

Related to #230