在多维数组中查找重复值

Question

I'm trying to code a system which will find user accounts that are using the same IPs, and print out the matches with their keys. I have already coded the part where the data is pulled from MySQL and nicely put in a multidimensional associative array, so input looks like this (the key is UserID, values are IP addresses):

$users = array(
               100 => array("1.1.1.1","2.2.2.2","3.3.3.3"),
               200 => array("1.1.1.1","4.4.4.4","5.5.5.5"),
               300 => array("1.1.1.1","4.4.4.4","7.7.7.7")
              );

The expected output would be:

Array
(
    [1.1.1.1] => Array
        (
            [0] => 100
            [1] => 200
            [2] => 300
        )

    [4.4.4.4] => Array
        (
            [0] => 200
            [1] => 300
        )
)

I have searched and used trial and error with multiple nested foreach that would loop through the arrays, tried array_intersect to find the duplicates, but nothing gets me close to the expected output. By now I think I'm overthinking the problem and the solution is really easy, but I cannot seem to get close to the expected output. Any help is appreciated, thank you.

Answer 1

$output = array();
// loop through each user
foreach ($users as $id => $ips) {
    // loop through each IP address
    foreach ($ips as $ip) {
        // add IP address, if not present
        if (!isset($output[$ip])) {
            $output[$ip] = array();         
        }
        // add user ID to the IP address' array
        $output[$ip][] = $id;
    }
}