PHP: Array Advanced & Sorting

Q: How do I choose between array_merge and the + operator?

Use array_merge in most cases. For merging user config over defaults: $config = array_merge($defaults, $userConfig); — user settings override defaults. Only use + when you specifically need "keep existing keys and ignore duplicates."

In the last lesson, you learned what arrays are and how to create and traverse them. This lesson unlocks where PHP arrays truly shine — the wealth of array functions that let you write stunningly concise code.

1. Merging Arrays

PHP

<?php
$arr1 = ["a" => "apple", "b" => "banana"];
$arr2 = ["b" => "blueberry", "c" => "cherry"];

// array_merge: merge — same-name string keys: latter overwrites former
$merged = array_merge($arr1, $arr2);
// ["a" => "apple", "b" => "blueberry", "c" => "cherry"]

// + operator: merge — same-name keys: former takes priority (keeps left side)
$union = $arr1 + $arr2;
// ["a" => "apple", "b" => "banana", "c" => "cherry"]
// Note: "b" kept "banana" from the left side

// array_merge re-indexes numeric arrays
$nums1 = [1, 2, 3];
$nums2 = [4, 5];
print_r(array_merge($nums1, $nums2));  // [1,2,3,4,5] (re-indexed)
print_r($nums1 + $nums2);              // [1,2,3] (+ doesn't re-index)
?>

	`array_merge`	`+` Operator
Same string keys	Latter overwrites former	Former overwrites latter
Indexed arrays	Re-indexes	Doesn't re-index; ignores existing indices

💡 Tip: To merge user-submitted data with default configuration, use $config = array_merge($defaults, $userInput); — the user's values override the defaults.

2. Array Difference and Intersection

PHP

<?php
$a = [1, 2, 3, 4, 5];
$b = [4, 5, 6, 7];

// array_diff: values in $a but not in $b
print_r(array_diff($a, $b));  // [1, 2, 3]

// array_intersect: values in both $a and $b
print_r(array_intersect($a, $b));  // [4, 5]
?>

3. Key and Value Operations

PHP

<?php
$user = ["name" => "John", "age" => 25, "city" => "New York"];

// array_keys: get all keys
print_r(array_keys($user));
// ["name", "age", "city"]

// array_values: get all values (reset to indexed array)
print_r(array_values($user));
// ["John", 25, "New York"]

// Check key or value
var_dump(array_key_exists("age", $user));  // bool(true)
var_dump(in_array("Chicago", $user));      // bool(false)

// array_search: find the key for a given value
$key = array_search(25, $user);
echo $key;  // "age"
?>

4. array_filter — Filtering Arrays

Keep elements based on whether a callback returns true or false:

PHP

<?php
$numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

// Keep only even numbers
$even = array_filter($numbers, function($n) {
    return $n % 2 == 0;
});
print_r($even);  // [1=>2, 3=>4, 5=>6, 7=>8, 9=>10]
// Note: the keys are preserved! Use array_values to reset them:
print_r(array_values($even));  // [2, 4, 6, 8, 10]

// No callback: filters out values that equal false
$mixed = [0, "hello", "", null, false, "world", []];
$filtered = array_filter($mixed);
print_r($filtered);  // [1=>"hello", 5=>"world"]
?>

▶ Example: Filtering Empty Form Data

Output:

TEXT 📖 Display only

value

PHP

<?php
// Clean up a user-submitted form (remove empty fields)
$form = [
    "name" => "John",
    "email" => "",
    "phone" => null,
    "city" => "New York",
    "comment" => false
];

$cleaned = array_filter($form, function($val) {
    return $val !== "" && $val !== null && $val !== false;
});
print_r($cleaned);
// ["name"=>"John", "city"=>"New York"]
?>

Output:

TEXT 📖 Display only

value
value
value

5. array_map — Applying a Function to Every Element

PHP

<?php
$numbers = [1, 2, 3, 4, 5];

// Multiply every number by 2
$doubled = array_map(function($n) {
    return $n * 2;
}, $numbers);
print_r($doubled);  // [2, 4, 6, 8, 10]

// With an arrow function
$squares = array_map(fn($n) => $n * $n, $numbers);
print_r($squares);  // [1, 4, 9, 16, 25]

// Process multiple arrays simultaneously
$a = [1, 2, 3];
$b = [4, 5, 6];
$sums = array_map(fn($x, $y) => $x + $y, $a, $b);
print_r($sums);  // [5, 7, 9]
?>

6. The Complete Sorting Family

PHP has 11 sorting functions, but you only need to remember 4 pairs + 1 custom:

PHP

<?php
$fruits = ["orange", "apple", "banana"];
$ages   = ["Peter" => 32, "John" => 28, "Jane" => 35];

// === Sort by Value ===

sort($fruits);   // Ascending, re-indexes → ["apple","banana","orange"]
rsort($fruits);  // Descending, re-indexes → ["orange","banana","apple"]

// === Sort by Value, Preserve Keys ===

asort($ages);    // Values ascending, preserve keys → ["John"=>28,"Peter"=>32,"Jane"=>35]
arsort($ages);   // Values descending, preserve keys → ["Jane"=>35,"Peter"=>32,"John"=>28]

// === Sort by Key ===

ksort($ages);    // Keys ascending → ["Jane"=>35,"John"=>28,"Peter"=>32]
krsort($ages);   // Keys descending → ["Peter"=>32,"John"=>28,"Jane"=>35]

// === Custom Sort ===

usort($fruits, fn($a, $b) => strlen($a) <=> strlen($b));
// Sort by string length ascending
?>

Function	Sorts By	Direction	Preserves Keys?
`sort`	Value	Ascending	❌ Re-indexes
`rsort`	Value	Descending	❌
`asort`	Value	Ascending	✅
`arsort`	Value	Descending	✅
`ksort`	Key	Ascending	✅
`krsort`	Key	Descending	✅
`usort`	Custom	Custom	❌
`uasort`	Custom (value)	Custom	✅

💡 Tip: asort / ksort preserve key-value relationships — use them for associative arrays (e.g., grade sheets). sort / rsort re-index — use them for plain lists.

7. Multi-Dimensional Arrays

Arrays within arrays — typically used to store structured data:

PHP

<?php
$students = [
    ["name" => "Alice", "score" => 92, "city" => "New York"],
    ["name" => "Bob", "score" => 85, "city" => "Chicago"],
    ["name" => "Charlie", "score" => 78, "city" => "New York"],
];

// Access: outer first, then inner
echo $students[0]["name"];  // Alice

// Traverse
foreach ($students as $s) {
    echo "{$s['name']}: {$s['score']} points<br>";
}

// Extract a single column with array_column
$names = array_column($students, "name");
print_r($names);  // ["Alice", "Bob", "Charlie"]

$scores = array_column($students, "score", "name");
// ["Alice"=>92, "Bob"=>85, "Charlie"=>78]
// The third argument specifies which column to use as keys
?>

▶ Example: Sorting Multi-Dimensional Arrays

Output:

TEXT 📖 Display only

{value['name']}: \${value['price']}<br>

PHP

<?php
$products = [
    ["name" => "Keyboard", "price" => 299],
    ["name" => "Mouse", "price" => 149],
    ["name" => "Monitor", "price" => 1299],
    ["name" => "Mousepad", "price" => 29],
];

// Sort by price low to high (custom comparison function)
usort($products, fn($a, $b) => $a["price"] <=> $b["price"]);

foreach ($products as $p) {
    echo "{$p['name']}: \${$p['price']}<br>";
}
// Mousepad: $29
// Mouse: $149
// Keyboard: $299
// Monitor: $1299
?>

Output:

TEXT 📖 Display only

Output displayed

The <=> spaceship operator is a perfect fit here — one line sorts a multi-dimensional array. Pure elegance.

8. Array Destructuring (PHP 7.1+)

Use [] or list() to unpack array values into multiple variables in one shot:

PHP

<?php
// Destructure an indexed array
$info = ["John", 25, "New York"];
[$name, $age, $city] = $info;
echo $name;  // John

// Destructure an associative array (PHP 7.1+)
$user = ["name" => "Jane", "age" => 22];
["name" => $n, "age" => $a] = $user;
echo $n;  // Jane

// Selective extraction (PHP 7.1+)
[2 => $city, 0 => $name] = $info;
echo $city;  // New York
?>

▶ Example: Chaining array_map, array_filter, and array_reduce

Output:

TEXT 📖 Display only

Alice, Bob, Charlie<br>
Total revenue: \${99.99}

PHP

<?php
$orders = [120, 45, 0, 85, 0, 200, 30];

$total = array_reduce(
    array_filter($orders, fn($o) => $o > 0),
    fn($sum, $o) => $sum + $o,
    0
);

$labels = array_map(fn($o) => $o > 0 ? "\${$o}" : "Free", $orders);
echo implode(", ", $labels) . "<br>";
echo "Total revenue: \${$total}";

Output:

TEXT 📖 Display only

Output displayed

❓ FAQ

Q How do I choose between array_merge and the + operator?

A Use array_merge in most cases. For merging user config over defaults: $config = array_merge($defaults, $userConfig); — user settings override defaults. Only use + when you specifically need "keep existing keys and ignore duplicates."

Q usort is powerful — when should I actually use it?

A When you need custom sorting rules — e.g., sort by price, sort by name length, sort by a deeply nested attribute. usort + <=> are a golden pair.

Q My multi-dimensional array is getting too deep — what should I do?

A Beyond 3 levels of nesting, consider using objects or classes to organize your data instead. Arrays are great for simple structures; objects are clearer for complex ones.

📖 Summary

array_merge merges arrays (latter overwrites); + merges (former takes priority)
array_filter filters by condition; array_map transforms every element
array_keys / array_values extract keys or values; in_array checks values
sort / asort / ksort and 3 more cover most sorting needs
usort + <=> for custom sorting — a multi-dimensional array sorting powerhouse
array_column extracts a specific column from multi-dimensional arrays
Array destructuring [$a, $b] = $arr for concise assignment

📝 Exercises

Given two arrays of user tags, use array_merge to combine them and array_unique to deduplicate the result. (Hint: array_unique() removes duplicates.)
From a scores array [85, 92, 78, 95, 60]: use array_filter to keep only passing scores (≥ 60), then use array_map to add 5 bonus points to each, and finally sort and output the result.
Create a multi-dimensional array with at least 5 books (title, author, price), use usort to sort by price low to high, then use array_column to extract all author names.

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PHP: Array Advanced & Sorting

1. Merging Arrays

2. Array Difference and Intersection

3. Key and Value Operations

4. array_filter — Filtering Arrays

▶ Example: Filtering Empty Form Data

5. array_map — Applying a Function to Every Element

6. The Complete Sorting Family

7. Multi-Dimensional Arrays

▶ Example: Sorting Multi-Dimensional Arrays

8. Array Destructuring (PHP 7.1+)

▶ Example: Chaining array_map, array_filter, and array_reduce

❓ FAQ

📖 Summary

📝 Exercises

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