Add PHP array functions tutorial and example

data/04-arrays/examples.php

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+<?php
+
+$prices = [10, 20, 30, 40, 50];
+
+// Use array_filter to get prices above 25
+$expensive = array_filter($prices, function($price) {
+    return $price > 25;
+});
+
+// Use array_map to add 10% tax to each price
+$withTax = array_map(fn($p) => $p * 1.10, $prices);
+
+print_r($expensive);
+print_r($withTax);
+
+?>

data/04-arrays/guide.md

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+# Modern PHP Array Manipulation
+
+PHP 8.x offers several powerful functions to handle arrays without using messy `foreach` loops for every operation.
+
+### Key Takeaways
+* **`array_map`**: Applies a callback to every element in the array.
+* **`array_filter`**: Filters elements based on a specific boolean condition.
+* **Cleaner Code**: Using these functions makes your backend logic more functional and less prone to "off-by-one" errors.