The mapWithKeys method iterates through the collection and passes each value to the given callback. The callback should return an associative array containing a single key / value pair:

$collection = collect([
    [
        'name' => 'John',
        'department' => 'Sales',
        'email' => 'john@example.com',
    ],
    [
        'name' => 'Jane',
        'department' => 'Marketing',
        'email' => 'jane@example.com',
    ]
]);
 
$keyed = $collection->mapWithKeys(function ($item, $key) {
    return [$item['email'] => $item['name']];
});
 
$keyed->all();
 
/*
    [
        'john@example.com' => 'John',
        'jane@example.com' => 'Jane',
    ]
*/