Exception Classes

Part of the Object Oriented Programming section of Coddy's PHP journey — lesson 66 of 91.

In PHP, exceptions are objects used to signal that something went wrong during execution. The base class for all exceptions is Exception, and understanding its structure helps you handle errors effectively in your OOP applications.

When you throw an exception, you create an instance of the Exception class (or a subclass) with a message describing what went wrong:

<?php
class BankAccount {
    public function __construct(private float $balance = 0) {}
    
    public function withdraw(float $amount): float {
        if ($amount > $this->balance) {
            throw new Exception("Insufficient funds");
        }
        $this->balance -= $amount;
        return $amount;
    }
}

$account = new BankAccount(100);
$account->withdraw(150);

This would produce an uncaught exception error with the message "Insufficient funds".

PHP provides several built-in exception classes for different situations. InvalidArgumentException signals bad input, RuntimeException indicates errors during execution, and LogicException represents programming errors:

<?php
class Calculator {
    public function divide(int $a, int $b): float {
        if ($b === 0) {
            throw new InvalidArgumentException("Cannot divide by zero");
        }
        return $a / $b;
    }
}

Every exception object carries useful information: getMessage() returns the error message, getCode() returns an optional error code, getFile() and getLine() tell you where the exception was thrown. Using specific exception types makes your code's intent clearer and allows for more precise error handling.

Challenge

Easy

Let's build an inventory management system that uses PHP's built-in exception classes to signal different types of problems. You'll create a product inventory where operations can fail in specific, meaningful ways.

You'll organize your code across three files:

Product.php — Create a Product class that represents an item in inventory. Use constructor promotion for a public $sku (string), a public $name (string), and a private $quantity (int). Your class should have:
- A method getQuantity(): int that returns the current quantity
- A method addStock(int $amount): void that increases the quantity. If the amount is less than 1, throw an InvalidArgumentException with the message "Amount must be positive"
- A method removeStock(int $amount): void that decreases the quantity. If the amount is less than 1, throw an InvalidArgumentException with the message "Amount must be positive". If the amount exceeds the current quantity, throw a RuntimeException with the message "Insufficient stock"
Inventory.php — Create an Inventory class that manages a collection of products. Include the Product file. Your class should have:
- A private array to store products indexed by SKU
- A method addProduct(Product $product): void that adds a product. If a product with the same SKU already exists, throw a LogicException with the message "Product already exists: [sku]"
- A method getProduct(string $sku): Product that returns a product by SKU. If not found, throw an InvalidArgumentException with the message "Product not found: [sku]"
main.php — Include the Inventory file. You'll receive three inputs: a SKU, a product name, and an operation ("add", "remove", or "duplicate").
Create an Inventory and add a new Product with the given SKU, name, and an initial quantity of 10.

Based on the operation:
- "add": Call addStock(5) on the product, then print "Stock updated: [quantity]"
- "remove": Call removeStock(15) on the product (this should throw an exception)
- "duplicate": Try to add another product with the same SKU (this should throw an exception)
When an exception is thrown, print the exception's class name followed by ": " and the exception's message. Use PHP's get_class() function to get the class name.

This challenge demonstrates how different exception types communicate different kinds of problems — invalid input, runtime failures, and logic errors — making your code's error handling more precise and meaningful.

Cheat sheet

In PHP, exceptions are objects that signal errors during execution. The base class is Exception.

To throw an exception, create an instance with a descriptive message:

<?php
class BankAccount {
    public function __construct(private float $balance = 0) {}
    
    public function withdraw(float $amount): float {
        if ($amount > $this->balance) {
            throw new Exception("Insufficient funds");
        }
        $this->balance -= $amount;
        return $amount;
    }
}

PHP provides built-in exception classes for different situations:

InvalidArgumentException — signals bad input
RuntimeException — indicates errors during execution
LogicException — represents programming errors

<?php
class Calculator {
    public function divide(int $a, int $b): float {
        if ($b === 0) {
            throw new InvalidArgumentException("Cannot divide by zero");
        }
        return $a / $b;
    }
}

Exception objects provide useful methods:

getMessage() — returns the error message
getCode() — returns an optional error code
getFile() — returns the file where the exception was thrown
getLine() — returns the line number where the exception was thrown

Use get_class() to retrieve the class name of an exception object.

Try it yourself

<?php

require_once 'Inventory.php';

// Read inputs
$sku = trim(fgets(STDIN));
$name = trim(fgets(STDIN));
$operation = trim(fgets(STDIN));

try {
    // TODO: Create an Inventory
    // TODO: Create a Product with the given SKU, name, and initial quantity of 10
    // TODO: Add the product to the inventory
    
    // TODO: Based on the operation:
    // - "add": Call addStock(5) on the product, then print "Stock updated: [quantity]"
    // - "remove": Call removeStock(15) on the product
    // - "duplicate": Try to add another product with the same SKU
    
} catch (Exception $e) {
    // TODO: Print the exception class name followed by ": " and the message
    // Hint: Use get_class($e) to get the class name
}

?>