How do I count the values of a specific key in a multi-dimensional array in PHP?

ok say the array is called $store

$count = 0;
$s = array();

foreach($store as $store){
 $count = 0;
 $count += $store['apples'];
 $count += $store['oranges'];

 $s[$store['store']] = $count;
}

add more fruit if needed.

This is a more complicated example, but better than spaghetti code. Turn your array into XML first.

// this is your initial array
$my_array = array(
array("city"=>309, "store"=>12, "apples"=>21, "oranges"=>14, "lichis"=>34 ),
array("city"=>309, "store"=>13, "apples"=>0, "oranges"=>11, "lichis"=>32 ),
array("city"=>309, "store"=>14, "apples"=>44, "oranges"=>61, "lichis"=>0 ),
array("city"=>309, "store"=>15, "apples"=>7, "oranges"=>0, "lichis"=>6 ),
array("city"=>309, "store"=>16, "apples"=>0, "oranges"=>0, "lichis"=>12 ),
);

Writing spaghetti code to manually isolate stores becomes a messy jungle of if statements and loops.

// might as well convert it to xml
function array_to_xml( $data ) {
    $xml = "<xml>\n";
    foreach( $data as $row ) {
        $xml .= "<entry>";
        foreach( $row as $k => $v ) {
            $xml .= "<{$k}>{$v}</{$k}>";
        }
        $xml .= "</entry>\n";
    }
    $xml .= "</xml>\n";
    return( $xml );
}

At this point, $xml looks like this (as a string) and is more manageable:

<xml>
  <entry>
    <city>309</city>
    <store>12</store>
    <apples>21</apples>
    <oranges>14</oranges>
    <lichis>34</lichis>
  </entry>
  ...(more entries)...
</xml>

Now, load it into something queriable with XPath, an XML standard:

$xml = simplexml_load_string( array_to_xml( $my_array ) );

To get the count of all the specific fruits in the city (i.e. how many apples, oranges, lichies are there in total in city 309) we need a simple but reusable summary function.

// so lets make a generic function to count specific items
function count_items( $stores, $items = array() ) {
    $sum = array();
    foreach( $stores as $store ) {
        foreach( $items as $k ) {
            if( !isset( $sum[$k] ) ) $sum[$k] = 0;
            $sum[$k] += $store->$k;
        }
    }
    return( $sum );
}

We only want city 309, and looking specifically for apples, oranges and lichis, since they are fruits:

$only_this_city = $xml->xpath("//entry[city=309]");
print_r( count_items( $only_this_city, array("apples", "oranges", "lichis")) );

We get this:

Array
(
    [apples] => 72
    [oranges] => 86
    [lichis] => 84
)

Secondly, to grab the values for a specific store:

$only_this_store = $xml->xpath("//entry[city=309 and store=14]");
print_r( count_items( $only_this_store, array("apples") ) );

You get:

Array
(
    [apples] => 44
)

Obviously you can request more items, or query with more complexity. Look up some docs on XPath for future queries.

function extractData( array $data, $store = null )
{
    $callback = function( $values, $array ) use ( $store )
    {
        // We require values for a particular store
        if( ! is_null( $store ) )
        {
            if( $array[ 'store' ] == $store )
            {
                return $array;
            }

            return false;
        }
        // Return the sum of all stores in a city
        else
        {
            // Toss out the city and store values since they're not needed
            unset( $array['city'], $array['store'] );

            // Seed the initial values
            if( ! is_array( $values ) || empty( $values ) )
            {
                return $array;
            }

            // array_map function used add the arrays
            $add = function( $a, $b )
            {
                return $a + $b;
            };

            // Add the arrays
            $summedArray = array_map( $add, $values, $array );

            // Return the combined array structure
            return array_combine( array_keys( $values ), $summedArray );
        }
    };

    return array_reduce( $data, $callback, array() );
}

$data = array(
    0 => array(
        "city" => 309,
        "store" => 12,
        "apples" => 21,
        "oranges" => 14,
        "lichis" => 34
    ),
    1 => array(
        "city" => 309,
        "store" => 13,
        "apples" => 0,
        "oranges" => 11,
        "lichis" => 32
    ),
    2 => array(
        "city" => 309,
        "store" => 14,
        "apples" => 44,
        "oranges" => 61,
        "lichis" => 0
    ),
    3 => array(
        "city" => 309,
        "store" => 15,
        "apples" => 7,
        "oranges" => 0,
        "lichis" => 6
    ),
    4 => array(
        "city" => 309,
        "store" => 16,
        "apples" => 0,
        "oranges" => 0,
        "lichis" => 12
    )
);

// Return the values for a particular store
print_r( extractData( $data, 16 ) );

// Return the total of all stores in the city
print_r( extractData( $data ) );

Which would yield the following results...

Single City

Array
(
    [city] => 309
    [store] => 16
    [apples] => 0
    [oranges] => 0
    [lichis] => 12
)

Totals

Array
(
    [apples] => 72
    [oranges] => 86
    [lichis] => 84
)

NOTE: Before any reaction… I know the OP has not asked how to handle this in an object-oriented fashion so this is just a suggestion.

I would create a separate class for the overall data and another for each store data.

$arr = array(
    array('city' => 309, 'store' => 12, 'apples' => 21, 'oranges' => 14, 'lichis' => 34),
    array('city' => 309, 'store' => 13, 'apples' => 0, 'oranges' => 11, 'lichis' => 32),
    array('city' => 309, 'store' => 14, 'apples' => 44, 'oranges' => 61, 'lichis' => 0),
    array('city' => 309, 'store' => 15, 'apples' => 7, 'oranges' => 0, 'lichis' => 6),
    array('city' => 309, 'store' => 16, 'apples' => 0, 'oranges' => 0, 'lichis' => 12)
);

foreach ($arr as $rec) {
    $storeID = $rec['store'];
    $city = $rec['city'];
    unset($rec['city'],$rec['store']);
    // assuming all the keys except these 2
    // represent product name->quantity pairs
    StoreData::add(new Store($storeID,$city,$rec));
}
//print_r(StoreData::$stores);

echo
'<b>total products:</b> ' .
    StoreData::get_total_products() . PHP_EOL .
'<b>total products in city #309:</b> ' .
    StoreData::get_total_products_by_city(309) . PHP_EOL .
'<b>"apples" everywhere:</b> ' .
    StoreData::get_product_total('apples') . PHP_EOL .
'<b>"apples" in store #12:</b> ' .
    StoreData::get_product_total_by_store('apples',12) . PHP_EOL .
'<b>"apples" in city #309:</b> ' .
    StoreData::get_product_total_by_city('apples',309);


class StoreData {

    public static $stores = array();

    public static function add(Store $store) {
        self::$stores[] = $store;
    }

    public static function remove(Store $store) {
        foreach (self::$stores as $key => $value) {
            if ($value == $store) {
                unset(self::$stores[$key]);
                break;
            }
        }
    }

    public static function find_by_city($cityID) {
        $cityID = (int) $cityID;
        return array_filter(self::$stores,create_function(
            '$store',
            "return \$store->city == $cityID;")
        );
    }

    public static function find_by_store($storeID) {
        $storeID = (int) $storeID;
        foreach (self::$stores as $store) {
            if ($store->id == $storeID) return $store;
        }
        return FALSE;
    }

    public static function get_total_products() {
        $total = 0;
        foreach (self::$stores as $store)
            $total += $store->get_total_products();
        return $total;
    }

    public static function get_total_products_by_city($city) {
        $stores = self::find_by_city((int) $city);
        $total = 0;
        foreach ($stores as $store)
            $total += $store->get_total_products();
        return $total;
    }

    public static function get_product_total_by_city($productName,$city) {
        return self::product_total($productName,self::find_by_city((int) $city));
    }

    public static function get_product_total_by_store($productName,$storeID) {
        $res = self::find_by_store((int) $storeID);
        return $res ? self::product_total($productName,array($res)) : $res;
    }

    public static function get_product_total($productName) {
        return self::product_total($productName,self::$stores);
    }

    private static function product_total($productName,$stores=NULL) {
        $total = 0;
        foreach ($stores as $store)
            $total += $store->get_product_total($productName);
        return $total;
    }

}

class Store {

    public $id;
    public $city;
    public $products;

    function __construct($id,$city,$products=array()) {
        $this->id = $id;
        $this->city = $city;
        $this->products = $products;
    }

    public function get_total_products() {
        return array_sum($this->products);
    }

    public function get_product_total($productName) {
        if (isset($this->products[$productName]))
            return (int) $this->products[$productName];
        return 0;
    }

}

will output

total products: 242
total products in city #309: 242
"apples" everywhere: 72
"apples" in store #12: 21
"apples" in city #309: 72