<?php

$a1 = array("a" => "red", "a" => "red", "b" => "green", "c" => "blue", "d" => "yellow");
$a2 = array("e" => "red", "e" => "red", "f" => "green", "g" => "blue");

$result = array("d" => "yellow");

// 0. if no dupes return
// 1. take a1 and find duplicates
$a1dupes = array("a" => "red");
// 2. if a2 NOT include "red" return
// else if "red"<2 return
// result:
$result = array("red");


function findDupesInBothArrays($arr1, $arr2)
{
    $a1dupes = array();
    for ($i = 0; $i < count($arr1); $i++) {
        # code...
        $valueToCheck = $arr1[$i];
        for ($j = 0; $j < count($arr1); $j++) {
            # code...
            if ($i == $j) {
                return;
            }
            if ($valueToCheck == $arr1[$j]) {
                // $a1dupes[$key] = $valueToCheck;
                array_push($a1dupes, $valueToCheck);
            }
        }
    }

    // $a1dupes = array("a" => "red");
    for ($i = 0; $i < count($a1dupes); $i++) {
        # code...
        $valueToCheck = $a1dupes[$i];
        for ($j = 0; $j < count($arr2); $j++) {
            # code...
            if ($valueToCheck == $arr2[$j]) {
                // $a1dupes[$key] = $valueToCheck;
                array_push($result, $valueToCheck);
            }
        }
    }

    if (count($result)) {
        return $result;
    }

    return null;
}