In PHP a property declaration
may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.
As a consequence, you cannot assign lambdas, closures, new instances, function returns or expressions. While explained in the PHP Manual, we already have a number of duplicates asking for either of those declarations. I wonder whether we could/should merge a few of those into a Canonical, maybe rephrasing the original question to accommodate for all the metioned cases above.
See the list Merging Candidates