You have a list of potential values to return, dependent on a value to evaluate. A super common operation, right? You'll instantly think, chuck in an if statement, or even a switch, and call it a day - let's see an example. We want to return the relatives name based on the relation (Does anyone else forget names easily??):

$relative = $_GET['relative'];

if($relative == 'mother') {
    return 'Harriet';
}

if($relative == 'father') {
    return 'Robert';
}

if($relative == 'daughter') {
    return 'Rachel';
}

if($relative == 'grandmother') {
    return 'Audrey';
}

// imagine 10 more! 

This is a perfectly adequate solution: but you'd probably want to use a switch, to simplify it even further:

$relative = $_GET['relative'];

switch ($relative) {
    case 'mother':
        return 'Harriet';
        break;
    case 'father':
        return 'Robert';
        break;
    case 'daughter':
        return 'Rachel';
        break;
    case 'grandmother':
        return 'Audrey';
        break;
    default:
        return 'relative not found!';
}

Great, this feels much more structured, but it's quite heavy on syntax, right? Perfectly serviceable, but let's take it one step further.

Enter lookup-arrays. A lookup array is a key-value paired list, where the key is what is looked up!

The above, in the form of a lookup-array is:

$relative = $_GET['relative'];

$relatives = [
    'mother' => 'Harriet',
    'father' => 'Robert',
    'daughter' => 'Rachel',
    'grandmother' => 'Audrey',
];

return $relatives[$relative] ?? 'relative not found!';

You can see how much easier to read a lookup-array is, and how it dramatically reduces the quantity of code, in a concise way! To note, lookup-arrays aren't always the best choice, but when they are, it should feel pretty natural!

Thanks for reading - let me know your thoughts!