PHP/trait/use

About

This sense of the use keyword is for including traits; there's another sense that only relates to namespaces.

The use keyword allows a trait to be used within a class or another trait. There unfortunately does not seem to be a page specifically documenting its syntax; most (but not all) of the applicable syntax can be found on the traits documentation page.

Basic Form

trait tSample1 {}
class cSample1 { use tSample1; }
trait tSample2 { use tSample1; }

Form for Multiple Traits

Building on the above example:

class cSample2 { use tSample1, tSample2; }
// This ^ is equivalent to:
class cSample2 {
    use tSample1;
    use tSample2;
}

Form for Conflict Resolution

Starting afresh...

trait tSample1 {
    function Func1() {}
    function Func2() {}
}
trait tSample2 {
    function Func1() {}
    function Func2() {}
}
// If we want to use both tSample3 and tSample4, how do we indicate which version of Func1() to use?
// Here's the syntax for that:
class cSample1 {
    use tSample1, tSample2 {
        tSample2::Func1 insteadof tSample1;
        tSample1::Func2 insteadof tSample2;
    }
}

Form for Pseudoparent Calls

This is currently not documented on the traits page; I'm not sure where I found it.

Starting again with fresh example code, here is the problem:

trait tSample1 {
    function Func1() {}
}
trait tSample2 {
    use tSample1;
    function Func1() {
        parent::Func1();  // This doesn't work! A "use"d trait is not a parent.
    }
}

To fix this, alter tSample2 thusly:

trait tSample2 {
    use tSample1 { tSample1::Func1 as parent_Func1; }
    function Func1() {
        parent_Func1();  // We'll pretend we're a real family. (Are you my mummy?)
    }
}

In case you need to alias multiple functions (starting over):

// 2022-04-13 I reconstructed this syntax through trial-and-error, but I swear it's in the docs. Somewhere.
trait tSample1 {
    function Func1() {}
    function Func2() {}
    function Func3() {}
}
trait tSample2 {
    use tSample1 {
      tSample1::Func1 as parent_Func1;
      tSample1::Func2 as parent_Func2;
      tSample1::Func3 as parent_Func3;
    }
    function Func1() { $this->parent_Func1(); }
    function Func2() { $this->parent_Func2(); }
    function Func3() { $this->parent_Func3(); }
}

Other

You can also change the visibility (public/protected/private) of a function with "use", but I've never had occasion to... use... this feature.