Remove consecutive duplicates of a specified value from a 2d array

Please note that your sample output array has duplicated keys, which I think, is not allowed. Try this:

<?php

$out = [
    '2011' => ['Out', 'arrv'],
    '2012' => ['Out'],
    '2013' => ['Out'],
    '2014' => ['Out'],
    '2015' => ['Out'],
    '2016' => ['some', 'none', 'test'],
    '2017' => ['Out'],
    '2018' => ['Out'],
    '2019' => ['Out'],
    '2020' => ['Out', 'did'],
    '2021' => ['Out'],
    '2022' => ['Out'],
    '2023' => ['Out', 'did']
];

$cleaned_out = [];
$first = true;
$last_key = null;
$last_value = null;
$key = null;
$value = null;
$printed_last_key = false;

foreach ($out as $key => $values) {
    foreach ($values as $value) {
        if ($first) {
            $last_key = $key;
            if ($value == 'Out') {
                $first = false;
            }
            echo $key . ' ' . $value . PHP_EOL;
            $printed_last_key = true;
        } else {
            if ($value == 'Out') {
                $last_key = $key;
                $last_value = $value;
                $printed_last_key = false;
            } else {
                if (!$printed_last_key) {
                    echo $last_key . ' ' . $last_value . PHP_EOL;
                }
                echo $key . ' ' . $value . PHP_EOL;
                $cleaned_out[$last_key] = $last_value;
                $cleaned_out[$key] = $value;
                $first = true;
                $printed_last_key = true;
            }
        }
    }
}

?>

As you iterate over this associative array of rows, maintain a counter to allow you to check if the previous, current, and next element is explicitly ['Out'].

If so, then print nothing. Otherwise, print each value in the row with the first level key.

Code: (Demo)

$i = -1;
foreach ($out as $k => $row) {
    ++$i;
    if (array_slice($out, $i - 1, 3) === [['Out'], ['Out'], ['Out']]) {
        continue;
    }
    foreach ($row as $v) {
        echo "$k: $v\n";
    }
}

2011: Out
2011: arrv
2012: Out
2015: Out
2016: some
2016: none
2016: test
2017: Out
2019: Out
2020: Out
2020: did
2021: Out
2022: Out
2023: Out
2023: did

Forgivably, the first iteration that subtracts 1 from 0 inside of array_slice() causes the sliced result to start from the back of the array (-1 means start the slice from the second last element). Because only a 3-element slice will be disqualified, this is safe operation. In other words, the first and last element of the array will never be disqualified.

Here is the same technique without manually incrementing the counter variable. Demo

foreach (array_keys($out) as $i => $k) {
    if (array_slice($out, $i - 1, 3) === [['Out'], ['Out'], ['Out']]) {
        continue;
    }
    foreach ($out[$k] as $v) {
        echo "$k: $v\n";
    }
}

If you are actually trying to "pop-out" these unwanted rows, then unset() row by their key. Demo

$i = 0;
foreach ($out as $k => $row) {
    if (array_slice($out, $i - 1, 3) === [['Out'], ['Out'], ['Out']]) {
        unset($out[$k]);
        continue;
    }
    ++$i;
}
var_export($out);

I'll ignore the error

array_keys() expects parameter 1 to be array, integer given

because it's only outputting some debugging information; what you need to resolve is the logic used to detect the "islands" in your sequence of years.

This code is not elegant, won't work for all combinations of the data, and doesn't split up the array data - as that's not the issue here - but hopefully it will help you:

$result = [];
$inSequence = false;
$prev = null;

foreach ($out as $year => $data) {
  if (str_contains($data[0], "Out")) {
    if (!$inSequence) {
      $result[$year] = $data; // Save first of sequence
      $inSequence = !$inSequence; // toggle as in sequence
    }
  } else {
    // have we interrupted the sequence, stash last 'Out' if we have one
    if ($prev) {
        $result[$prev] = $out[$prev];
        $inSequence = !$inSequence; // toggle out of a sequence
    }
    // add current data that does not start with 'Out'
    $result[$year] = $data;
  }
  // Remember current item in case we need it next time round, or to add it on the end.
  $prev = $year;
}

// You'll need to do some checking to see if you actually need to add this on the end, but for the example data we do.
$result[$prev] = $out[$prev];

print_r($result);

What this code does is to maintain a status of when we enter a sequence of "Out" values, and keeps track of the "last" item we looked at in case we need to insert it into the $result array because the current element is not an "Out".

The output of this code is:

Array
(
    [2011] => Array
        (
            [0] => Out
            [1] => arrv
        )
    [2015] => Array
        (
            [0] => Out
        )
    [2016] => Array
        (
            [0] => some
            [1] => none
            [2] => test
        )
    [2017] => Array
        (
            [0] => Out
        )
    [2023] => Array
        (
            [0] => Out
            [1] => did
        )
)

Let me know if you need anything clarified.

When I ran your code I see this output with error messages:

year = 2011, data = Array
(
    [0] => Out
    [1] => arrv
)

year = 2012, data = Array
(
    [0] => Out
)

2012 contains the string; index is 1

   WARNING  Undefined array key 0 in phar:///Applications/Tinkerwell.app/Contents/Resources/tinkerwell/tinker.phar/src/Shell/CustomExecutionLoopClosure.php(38) : eval()'d code on line 20.

   WARNING  Trying to access array offset on null in phar:///Applications/Tinkerwell.app/Contents/Resources/tinkerwell/tinker.phar/src/Shell/CustomExecutionLoopClosure.php(38) : eval()'d code on line 20.

   DEPRECATED  str_contains(): Passing null to parameter #1 ($haystack) of type string is deprecated 

... etc