Fix #81: Add article on immutability (#241)

* Fix #81: Add article on immutability

* Fix default examples

Author: samdark

guide/en/README.md

@@ -42,6 +42,7 @@ Key concepts +
 * [Configuration](concept/configuration.md) +
 * [Aliases](concept/aliases.md) +
 * [Events](concept/events.md) +
+* [Immutability](concept/immutability.md) +
 
 Handling requests +
 -----------------

guide/en/concept/immutability.md

@@ -0,0 +1,151 @@
+# Immutability
+
+Immutability means an object's state cannot change after it has been created. 
+Instead of modifying an instance, you create a new instance with the desired changes. 
+This approach is common for value objects such as Money, IDs, and DTOs. It helps to avoid accidental side effects:
+methods cannot silently change shared state, which makes code easier to reason about.
+
+## Mutable pitfalls (what we avoid)
+
+```php
+// A shared base query built once and reused:
+$base = Post::find()->where(['status' => Post::STATUS_PUBLISHED]);
+
+// Somewhere deep in the code we only need one post:
+$one = $base->limit(1)->one(); // mutates the underlying builder (sticky limit!)
+
+// Later we reuse the same $base expecting a full list:
+$list = $base->orderBy(['created_at' => SORT_DESC])->all();
+// Oops: still limited to 1 because the previous limit(1) modified $base.
+```
+
+## Creating an immutable object in PHP
+
+There is no direct way to modify an instance, but you can use clone to create a new instance with the desired changes.
+That is what `with*` methods do.
+
+```php
+final class Money
+{
+    public function __construct(
+        private int $amount,
+        private string $currency,
+    ) {
+        $this->validateAmount($amount);
+        $this->validateCurrency($currency);
+    }
+    
+    private function validateAmount(string $amount) 
+    {
+     if ($amount < 0) {
+            throw new InvalidArgumentException('Amount must be positive.');
+        }
+    }
+    
+    private function validateCurrency(string $currency)
+    {
+        if (!in_array($currency, ['USD', 'EUR'])) {
+            throw new InvalidArgumentException('Invalid currency. Only USD and EUR are supported.');
+        }
+    } 
+
+    public function withAmount(int $amount): self
+    {
+        $this->validateAmount($amount);
+    
+        if ($amount === $this->amount) {
+            return $this;
+        }
+    
+        $clone = clone $this;
+        $clone->amount = $amount;
+        return $clone;
+    }
+    
+    public function withCurrency(string $currency): self
+    {
+        $this->validateCurrency($currency);
+    
+        if ($currency === $this->currency) {
+            return $this;
+        }
+    
+        $clone = clone $this;
+        $clone->currency = $currency;
+        return $clone;
+    }
+    
+    public function amount(): int 
+    {
+        return $this->amount;
+    }
+    
+    public function currency(): string 
+    {
+        return $this->currency;
+    }
+
+    public function add(self $money): self
+    {
+        if ($money->currency !== $this->currency) {
+            throw new InvalidArgumentException('Currency mismatch. Cannot add money of different currency.');
+        }
+        return $this->withAmount($this->amount + $money->amount);
+    }
+}
+
+$price = new Money(1000, 'USD');
+$discounted = $price->withAmount(800);
+// $price is still 1000 USD, $discounted is 800 USD
+```
+
+- We mark the class `final` to prevent subclass mutations; alternatively, design for extension carefully.
+- Validate in the constructor and `with*` methods so every instance is always valid.
+
+> [!TIP]
+> If you define a simple DTO, you can use modern PHP `readonly` and leave properties `public`. The `readonly` keyword
+> would ensure that the properties cannot be modified after the object is created.
+
+## Using clone (and why it is cheap)
+
+PHP's clone performs a shallow copy of the object. For immutable value objects that contain only scalars 
+or other immutable objects, shallow cloning is enough and fast. In modern PHP, cloning small value objects is 
+inexpensive in both time and memory.
+
+If your object holds mutable sub-objects that must also be copied, implement `__clone` to deep-clone them:
+
+```php
+final class Order
+{
+    public function __construct(
+        private Money $total
+    ) {}
+    
+    public function total(): Money 
+    {
+        return $this->total;
+    }
+
+    public function __clone(): void
+    {
+        // Money is immutable in our example, so a deep clone is not required.
+        // If it were mutable, you could do: $this->total = clone $this->total;
+    }
+
+    public function withTotal(Money $total): self
+    {
+        $clone = clone $this;
+        $clone->total = $total;
+        return $clone;
+    }
+}
+```
+
+## Usage style
+
+- Build a value object once and pass it around. If you need a change, use a `with*` method that returns a new instance.
+- Prefer scalar/immutable fields inside immutable objects; if a field can mutate, isolate it and deep-clone in `__clone`
+  when needed.
+
+Immutability aligns well with Yii's preference for predictable, side-effect-free code and makes services, caching,
+and configuration more robust.