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Run php b.php; returns:
Fatal error: Declaration of myTrait::foo(): string must be compatible with a::foo(): int in .../b.php on line 19

Should return something like:
Fatal error: Declaration of myTrait::foo(): string must be compatible with a::foo(): int in .../b.php on line 5

Or:
Fatal error: Declaration of myTrait::foo(): string must be compatible with a::foo(): int in .../myTrait.php on line 19

Or:
Fatal error: Declaration of myTrait::foo(): string must be compatible with a::foo(): int in .../b.php on line 5, from trait on .../myTrait.php on line 19

a.php

<?php
class a {
    public function foo(): int {return 2;}
}

b.php

<?php
require_once "a.php";
require_once "myTrait.php";
class b extends a {
    use myTrait;
}

myTrait.php

<?php
// there
// are
// some
// lines
// here
// to
// show
// that
// the
// error
// uses
// this
// file's
// line
// number
trait myTrait
{
    public function foo(): string {return "oops";}
}