Nice!

Nice, cleaner than mine. I had one more capturing group that I now see I didn't need.

PHP test only throws:
Parse error: syntax error, unexpected '<', expecting end of file in /home/codewarrior/run.php on line 10
before my function is even called.

iykyk...

This sure is a long way to code $guess = true;

The expected array contains a match: [16, 16] that should be [17, 18]

Failed asserting that two arrays are identical.
Expected: Array &0 (
    0 => 1
    1 => 2
    2 => 3
    3 => 4
    4 => 5
    5 => 6
    6 => 7
    7 => 8
    8 => 9
    9 => 10
    10 => 11
    11 => 12
    12 => 13
    13 => 14
    14 => 15
    15 => 16
    16 => 16
    17 => 16
    18 => 19
    19 => 20
)
Actual  : Array &0 (
    0 => 1
    1 => 2
    2 => 3
    3 => 4
    4 => 5
    5 => 6
    6 => 7
    7 => 8
    8 => 9
    9 => 10
    10 => 11
    11 => 12
    12 => 13
    13 => 14
    14 => 15
    15 => 16
    16 => 17
    17 => 18
    18 => 19
    19 => 20
)

"... for an observed PIN with a length of 1 to 8 digits"