Константин Ильин

function r_array_search($needle, $haystack, $currentKey = "") {
    foreach ($haystack as $key => $value) {
        if (is_array($value)) {
            $nextKey = r_array_search($needle, $value, $currentKey . "[{$key}]");            
            if ($nextKey) {
                return $nextKey;
            }
        } else if ($value == $needle) {
            return is_numeric($key) ? $currentKey . "[{$key}]" : $currentKey . "['{$key}']";
        }
    }
    return false;
}

$GLOBAL = [
    3515 => [
        'sub_categories' => [
            22916 => [
                'sub_categories' => [
                    33221 => 'test'
                ]
            ],
        ]
    ],
    35215 => [
        'sub_categories' => [
            221916 => 'test2'
        ]
    ],
];

print_r(r_array_search("test", $GLOBAL));

//[3515][sub_categories][22916][sub_categories][33221]