Reported by @driskell@github.com at https://github.com/wikimedia/less.php/issues/77 (26 April 2022):

Single and double quote are treated as different identifiers. This is contrast to javascript based LESS that treats a quoted identifier as a quoted identifier regardless of whether the quotes were single or double. It can cause a little confusion.

As an example:

.test-mixin(@value) when (@value = "test") {
  background: red;
}

.test-mixin(@value) when (@value = 'test') {
  background: blue;
}

double {
  .test-mixin("test")
}
single {
  .test-mixin('test')
}

Using wikimedia/less.php this compiles to the following:

double {
  background: red;
}
single {
  background: blue;
}

However, using lessc Node module you get:

double {
  background: red;
  background: blue;
}
single {
  background: red;
  background: blue;
}

Using versions:

$ lessc --version
lessc 4.1.2 (Less Compiler) [JavaScript]
$ ./vendor/bin/lessc --version
lessc 3.1.0 (less.php)