php85-nullkey-min.php (1,100 bytes)   

<?php

error_reporting(0);


function leaf(array $index)         // by-value copy-on-write array param
{
    $index[null] = true;            // emits E_DEPRECATED on PHP 8.5

    return $index;
}

if ($argc != 1) {
echo "attach error handler\n";
set_error_handler(static function ($errno, $errstr) {
    try {
        throw new Exception('handled');   // exception machinery during the deprecation
    } catch (\Throwable $e) {
        // swallow
    }

    return true;
});

}

$lost = 0;
$kept = 0;
$firstSamples = [];

for ($i = 0; $i < 100; $i++) {
    $src = [];
    $keys = array_keys(leaf($src['missing'] ?? []));   // `?? []` => fresh COW empty array
    if ($keys === []) {
        $lost++;
    } else {
        $kept++;
    }
    if ($i < 8) {
        $firstSamples[$i] = $keys;
    }
}

echo json_encode([
    'php'           => PHP_VERSION,
    'iterations'    => 100,
    'lost_dropped'  => $lost,   // how many times `$arr[null]=true` produced []
    'kept_ok'       => $kept,   // how many times it produced [""]
//    'first_samples' => $firstSamples,
], JSON_PRETTY_PRINT), PHP_EOL;