Question

I am trying to create a function to initiate date_compare() which is a usort function for a specific array and key.

function init_date_compare($key, $array) {
    $key2 = $key;
    function date_compare($a, $b) {
        global $key2;
        $t1 = strtotime($a[$key2]); $t2 = strtotime($b[$key2]);
        return $t2 - $t1;
    }
    usort($array, "date_compare");
}
$arr = array(array("Aug-2-2012"), array("June-2-2012"));
$arr = init_date_compare(0, $arr);
print_r($arr);

This outputs:

Notice: Undefined index: in...

(So basically null, the scoping did not work).

I am not sure how scoping works with functions inside functions, but if I remember right, it is possible. I tried throwing in some globals and initializing $key2 but I am unable to get this to work.

Answer 1

The reason your code isn’t working is because global $key2; inside the date_compare() function will not look for it inside the scope init_date_compare(); rather it will expect to find it inside the global scope.

Besides that, either the array should be passed by reference (i.e. &$array) via the function parameters, or the array should be returned from it.

Closures would make this a whole lot nicer (PHP >= 5.3):

function init_date_compare($key, &$array)
{
    usort($array, function(array $a, array $b) use ($key) {
        $t1 = strtotime($a[$key]); $t2 = strtotime($b[$key]);
        return $t2 - $t1;
    });
}

Another way is by using an object to encapsulate the state:

class DateComparer
{
    private $key;

    public function __construct($key)
    {
        $this->key = $key;
    }

    public function compare(array $a, array $b)
    {
        $t1 = strtotime($a[$this->key]); $t2 = strtotime($b[$this->key]);
        return $t2 - $t1;
    }
}

function init_date_compare($key, &$array)
{
    usort($array, array(new DateComparer($key), 'compare'));
}