In the interest of clarity and maintainability I would like to be able
to write code that makes it clear what kind of arguments a function
expects and what it returns.

This is what I tried:

function integer int_func(string $s) {
 // does something like, say, converting "five" to 5

There are two problems:
1 The appearance of a type name before the function name is treated as
a syntax error
2 Even if I forget about declaring the return type and code it instead

function int_func(string $s) {

I get a run-time error when I call the function with a string. (eg
$var = int_func("five");) The error message says"Catchable fatal
error: Argument 1 passed to int_func() must be an instance of string,
string given".

It seems that basic data types cannot be specified in ths way although
(intstances of) classes can. I have successfully used the technique to
catch run-time errors of wrong object types when testing, but am
surprised that I can't use it to trap unexpected basic types - or at
least to document what is expected.

To confuse me a bit further, I can't find a definitive list of the
basic type names. For example, is it "integer" or "int"?

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