What does "What use is X" mean?
I've come across this construct on Stack Overflow, in the title of the question "What use is lambda in PHP?"
To me this was an error, so I edited the question changing the title to "What is lambda used for in PHP?" and corrected some other stuff on the question.
Later the original poster changed the title back saying it was not an error and that it changed the voice of the phrase.
I'm not a native English speaker but consider myself pretty fluent in the language, and I have never seen this construct. Not that I remember, at least.
What does it mean?
The meaning doesn't change, but there is a subtle difference in the nuance, or "voice of the phrase".
What use is lambda in PHP?
Sounds to me like the speaker is skeptical of the benefits of using lambda in PHP. They probably know how to create solutions using different methods and don't need to use it, but they are curious about why others would use it instead of something else, or perhaps in a blunt manner, the speaker wants to know why anyone would use lambda instead of using whatever the speaker uses. "What use is..." sounds somewhat negative/pessimistic.
What is lambda used for in PHP?
Sounds more neutral and suggests the speaker is looking for a general run-down of everything that lambda can be used for in PHP. Not necessarily pros or cons, just what parts of PHP it is generally used for. It doesn't suggest that they use alternatives, or that they know of alternatives to using lambda.
In this case, 'use' is a noun meaning the value or advantage of something (not a verb, as you thought in your correction). So essentially: "what's the usefulness of lambda in PHP?"
One of the definitions in Oxford for use as a noun is
the value or advantage of something:
it was no use trying to persuade her
what’s the use of crying?
A related definition in the same source is
a purpose for or way in which something can be used:
the herb has various culinary uses
Its use in that sentence is perfectly fine.
One thing that nobody's mentioned before is that, while
the verb use is pronounced /yuz/, with a /z/, in all its verbal forms,
the noun use is pronounced /yus/, with an /s/, in all its nominal forms.
So the clause What use is lambda is pronounced /wət 'yusəz 'læmdə/
and not /wət 'yuzəz 'læmdə/, which would be What uses lambda.
If your mind's ear hears the consonants, there isn't a problem.
Rather like the rule for a ~ an or /ðə ~ ði/