• "... to avoid merge conflicts on adjacent lines" (to make the comment a bit more descriptive)

What's fake about the file? Seems like a normal test file to me. Is it faking anything?

To me it is faking ini files created by VersionPress. There is no guid at the top of created file. But you are right that the file created is legit for the purpouse of this test.

• change method name

I see that you have decided not to use constants for merge driver types. I know that it is just for tests. Isn't it better to replace those strings with constants, so potential users will know exact values which can be used ?

I treat test code the same as production code - it needs to have the same quality - so it's not an excuse here. I genuinely don't know what's better here, these are just three strings so I chose to use them directly without any "abstraction" but maybe that's just my taste. What would @JanVoracek do?

I would probably use constants because of preventing typos.

Another note to the code: I would make the conditions simpler as the final driver can be decided much sooner.

Ok, I'll add constants. BTW string literal types are an amazing feature of TypeScript, exactly what I'd use here if PHP supported it.

About the conditions: I had a feeling they could be simplified but it didn't occur to me how. Can you draft a pseudo-code (or real code) here? I'll happily apply it.

Maybe something like this?

if ($driver === self::DRIVER_AUTO) {
    $driver = DIRECTORY_SEPARATOR == '\\' ? self::DRIVER_PHP : self::DRIVER_BASH;
}

if ($driver === self::DRIVER_BASH) {
    $mergeDriverScript = $pluginDir . '/src/Git/merge-drivers/ini-merge.sh';
    chmod($mergeDriverScript, 0750);
}

if ($driver === self::DRIVER_PHP) {
    $mergeDriverScript = '"' . PHP_BINARY . '" "' . $pluginDir . '/src/Git/merge-drivers/ini-merge.php' . '"';
}