PHP中array_filter过滤多维数组及多重复URL参数取值异常问题

Hey there! Let's tackle your two PHP issues one by one — they're both common gotchas, so you're definitely not alone here.

The reason you only get the last value when passing City=NY&City=SF is that PHP's default behavior for $_GET/$_POST overwrites duplicate parameter keys, keeping only the final one. Here are two ways to fix this:

Option 1: Adjust the Request Parameter Format #

If you have control over how the request is sent, modify the parameter to use array syntax. Change your URL to:
City[]=NY&Category=Population&City[]=SF

With this format, $_GET['City'] will automatically be an array containing both NY and SF, so you can loop through it or use it directly in your filtering logic.

Option 2: Manually Parse the Query String (If You Can't Change the Request) #

If you can't adjust the request format, you'll need to parse the raw query string yourself to capture all duplicate keys. Here's a simple function to do that:

function getMultiParams() {
    $queryString = $_SERVER['QUERY_STRING'];
    $params = [];
    
    foreach (explode('&', $queryString) as $pair) {
        if (empty($pair)) continue;
        
        list($key, $value) = explode('=', $pair, 2);
        $key = urldecode($key);
        $value = urldecode($value);
        
        if (isset($params[$key])) {
            // Convert existing value to array if it's not already
            if (!is_array($params[$key])) {
                $params[$key] = [$params[$key]];
            }
            $params[$key][] = $value;
        } else {
            $params[$key] = $value;
        }
    }
    return $params;
}

// Usage:
$requestParams = getMultiParams();
// $requestParams['City'] will now be ['NY', 'SF']

array_filter works great for one-dimensional arrays, but for multidimensional data (like your database results), you need to pass a custom callback function that tells PHP how to evaluate each sub-array.

Let's say your database returns data like this:

$dbResults = [
    ['City' => 'NY', 'Category' => 'Population', 'Value' => 8336817],
    ['City' => 'SF', 'Category' => 'Population', 'Value' => 883305],
    ['City' => 'LA', 'Category' => 'Area', 'Value' => 503]
];

Example 1: Filter by Category #

To get only rows where Category is Population:

$filtered = array_filter($dbResults, function($row) {
    return $row['Category'] === 'Population';
});

Example 2: Combine with Multiple City Values #

To filter rows that match any of your City parameters and the Population category:

$cities = $requestParams['City']; // From our earlier function
$filtered = array_filter($dbResults, function($row) use ($cities) {
    // Check if the row's City is in our list and Category matches
    return in_array($row['City'], $cities) && $row['Category'] === 'Population';
});

Calculating the Sum #

Once you have your filtered rows, you can calculate the total using array_reduce:

$totalPopulation = array_reduce($filtered, function($sum, $row) {
    return $sum + $row['Value'];
}, 0);

echo "Total Population: " . $totalPopulation; // Outputs 9220122

内容的提问来源于stack exchange，提问作者noob92