Milestone: Control Flow

This milestone checks whether the first pieces of PHP are starting to work together. You have seen values, operators, branches, loops, functions, scope, and basic failure handling. Real PHP work combines those ideas instead of using them one at a time.

The goal is not to build a full application yet. The goal is to read a small piece of business logic, predict the path it takes, and change it without breaking the edge cases.

A Small Order Validator

The example below validates a list of orders and prints which ones can move to shipping.

<?php

function isPaidOrder(array $order): bool
{
    return ($order['status'] ?? '') === 'paid';
}

function hasPositiveTotal(array $order): bool
{
    return ($order['total_cents'] ?? 0) > 0;
}

$orders = [
    ['id' => 701, 'status' => 'paid', 'total_cents' => 2500],
    ['id' => 702, 'status' => 'pending', 'total_cents' => 1800],
    ['id' => 703, 'status' => 'paid', 'total_cents' => 0],
];

foreach ($orders as $order) {
    if (!isPaidOrder($order)) {
        echo "Skip order {$order['id']}: not paid\n";
        continue;
    }

    if (!hasPositiveTotal($order)) {
        echo "Skip order {$order['id']}: invalid total\n";
        continue;
    }

    echo "Ship order {$order['id']}\n";
}

// Prints:
// Ship order 701
// Skip order 702: not paid
// Skip order 703: invalid total

This code uses several beginner skills at once. The functions name the checks. The foreach loop processes every order. The if statements decide which path each order takes. continue skips to the next order after a failure.

Why The Functions Help

The checks could be written directly inside the loop, but naming them makes the rule easier to review:

• isPaidOrder() answers one question;
• hasPositiveTotal() answers one question;
• the loop decides what to do with each answer.

This is the habit to build early. A function should hide repetitive detail, not hide important decisions.

Missing Keys And Defaults

The example uses ?? when reading status and total_cents. That prevents a missing key from causing a warning in this small script, and it gives the validation rule a safe fallback.

This does not mean missing data is fine. It means the code decides what missing data means. In this example, a missing status is not paid, and a missing total is not a positive total.

When To Throw Instead

Sometimes a failure is expected and should be reported, as with an unpaid order. Sometimes a failure means the caller gave the function invalid data. In those cases, throwing an exception can be clearer.

<?php

function requireOrderId(array $order): int
{
    if (!isset($order['id'])) {
        throw new InvalidArgumentException('Order ID is required.');
    }

    return $order['id'];
}

echo requireOrderId(['id' => 701]) . "\n";

// Prints:
// 701

Use exceptions for failures the current function cannot sensibly handle. Use return values or branches for ordinary business choices the caller expects.

What This Milestone Proves

You are ready to move on when you can:

• trace which branch runs for each input;
• explain why a loop skips or processes an item;
• split a repeated check into a named function;
• pass data into functions instead of relying on hidden globals;
• handle missing data deliberately;
• decide whether a failure should be a branch or an exception.