I've created a src and test file (phpunit) to illustrate the problem. Just copy, paste and execute tests!

FILE: DebugBacktraceExample.php (3 classes)

<?php

class TestObject {
    static $count = 0;

    public function _function(string $func, string $file = null, int $line = null) {
        return sprintf("%s:%s:%d", $func, basename($file), $line);
    }

    public function _whoIsCallingMe(string $func, string $file = null, int $line = null) {
        $i = -1; // Count calling occurences in line
        $line = trim(file($file)[$line - 1]); // Calling line
        $identity = ""; // Result

        // Extract function, then remove from line, until no longer possible
        while(($next = preg_match("/\\$([a-zA-Z0-9]+)->$func\(.*?\)/", $line, $matches)) !== false && $i !== self::$count) {
            $identity = $matches[1]; // Set as identity
            $line = str_replace($matches[0], "", $line); // Clear match
            $i++;
        }

        self::$count += 1; // Increase global counter

        return $identity;
    }

    public function __call($func, $args) {
        $args[1] = debug_backtrace()[0]["file"];
        $args[2] = debug_backtrace()[0]["line"];
        return call_user_func_array([$this, "_$func"], $args);
    }
}

class TestObjectCopy extends TestObject { }

class Imposter {
    public function whoIsCallingMe(string $func) {
        return "booh!";
    }
}


FILE: DebugBacktraceExampleTest.php (PHPUNIT)

<?php
use PHPUnit\Framework\TestCase;

final class DebugBacktraceExampleTest extends TestCase {
    # CASE(S): debug_backtrace line information is inconsistent for array input
    public function testFunction1() {
        $result = (new TestObject)->function(__FUNCTION__); // <- line goes here
        $expectation = "testFunction1:DebugBacktraceExampleTest.php:8";
        $this->assertEquals($expectation, $result);
    }

    public function testFunction2() {
        $result = (new TestObject)->function((function($func){
            return $func; 
        })(__FUNCTION__)); // <- line goes here
        $expectation = "testFunction3:DebugBacktraceExampleTest.php:15";
        $this->assertEquals($expectation, $result);
    }

    public function testFunctionArrayFailure() {
        $result = (new TestObject)->function(implode("", array(
            "", // <- line goes here ???
            __FUNCTION__
        ))); // <-- seems to be the right place?
        $expectation = "testFunctionArrayFailure:DebugBacktraceExampleTest.php:25"; // Fail
        $this->assertEquals($expectation, $result);
    }

    # GOAL: use debug_backtrace file:line information to identify what object calls the 'whoIsCallingMe' function
    # PROBLEM: whoIsCallingMe is called twice on the same line
    # SOLUTION: keep count of occurences already encountered in the same line (see TestObject::$count)
    public function testWhoIsCallingMe()
    {
        $testObject1 = new TestObject();
        $testObject2 = new TestObjectCopy();
        $result = $testObject1->whoIsCallingMe('whoIsCallingMe') . " - " . $testObject2->whoIsCallingMe('whoIsCallingMe');
        $expectation = "testObject1 - testObject2";
        $this->assertEquals($expectation, $result);
    }

    # GOAL: use debug_backtrace file:line information to identify what object calls the 'whoIsCallingMe' function
    # PROBLEM: debug_backtrace file:line cannot be used to identify the caller on a line unambiguously because we included an 'imposter' as illustrated below
    # SOLUTION: not possible ?
    # OPTION 1 (cheapest): add '(int) col' to debug_backtrace adding the (strpos) start position of the function call start
    # OPTION 2 (expensive): introduce DEBUG_BACKTRACE_PROVIDE_LOCAL_VARS flag to debug_backtrace, adding local vars to trace stack revealing key/values (similar to get_defined_vars()).
    public function testWhoIsCallingMeWithImposter()
    {
        $testObject1 = new TestObject();
        $testObject2 = new Imposter();
        $testObject3 = new TestObjectCopy();
        $result = $testObject1->whoIsCallingMe('whoIsCallingMe') . " - " . $testObject2->whoIsCallingMe('whoIsCallingMe') . " - " . $testObject3->whoIsCallingMe('whoIsCallingMe');
        $expectation = "testObject1 - booh! - testObject3"; // Fail, receives testObject1 - booh! - testObject2
        $this->assertEquals($expectation, $result);
    }

    protected function tearDown() : void 
    {
        TestObject::$count = 0; // Reset for tests testWhoIsCallingMe/testWhoIsCallingMeWithImposter
    }
}