PHP: Arrays — The Basics

Q: Why doesn't my foreach modify the original array?

Unless you use &$val for reference traversal, foreach ($arr as $val) gives $val as a copy of each array element. Modifying $val doesn't affect the original array.

A PHP array is not an ordinary array — it's a super data structure that acts as a list, a dictionary, and a set all in one. In PHP, arrays are the single data type you'll use most often, bar none.

1. What a PHP Array Really Is

A PHP array is essentially an ordered map. In plain English: it maps keys to values and remembers the insertion order.

PHP

<?php
// Indexed array: keys are automatically assigned numbers (0, 1, 2, ...)
$fruits = ["apple", "banana", "orange"];

// Associative array: keys are strings you specify
$user = [
    "name" => "John",
    "age" => 25,
    "city" => "New York"
];
?>

Language	List	Dictionary	PHP Array
Python	`list`	`dict`	`array` does both
JavaScript	`Array`	`Object` / `Map`	`array` does both
PHP	`array`	`array`	One structure handles everything

2. Creating Arrays

PHP offers two syntaxes for creating arrays. [] is the modern syntax (PHP 5.4+) — use this one:

PHP

<?php
// Recommended syntax (PHP 5.4+)
$arr1 = [];                       // Empty array
$arr2 = [1, 2, 3];                // Indexed array
$arr3 = ["a" => 1, "b" => 2];     // Associative array

// Legacy syntax (for compatibility with older code)
$arr4 = array();
$arr5 = array(1, 2, 3);
$arr6 = array("a" => 1, "b" => 2);
?>

💡 Tip: [] is 5 characters shorter than array(). There's no reason not to use it.

3. Accessing Array Elements

Use square brackets [] with a key to read or modify elements:

PHP

<?php
$fruits = ["apple", "banana", "orange"];

echo $fruits[0];     // apple (index starts at 0)
echo $fruits[1];     // banana
echo $fruits[2];     // orange

$fruits[1] = "strawberry"; // Modify the second element
echo $fruits[1];     // strawberry

// Access associative arrays by key name
$user = ["name" => "John", "age" => 25];
echo $user["name"];  // John
$user["age"] = 26;   // Update the age
?>

⚠️ Warning: Accessing a key that doesn't exist triggers Warning: Undefined array key. Get in the habit of checking with isset() or array_key_exists() first.

4. Adding and Removing Elements

PHP

<?php
// === Adding Elements ===

$arr = [1, 2];
$arr[] = 3;          // Append to the end (the simplest syntax)
// $arr is now [1, 2, 3]

// Or use array_push (convenient when adding multiple)
array_push($arr, 4, 5);
// $arr is now [1, 2, 3, 4, 5]

// Add with a specific key
$arr["key"] = "value";

// === Removing Elements ===

unset($arr[2]);      // Remove the element with index 2
// $arr is now [1, 2, 4, 5, "key"=>"value"]

// array_pop: remove and return the last element
$last = array_pop($arr);

// array_shift: remove and return the first element
$first = array_shift($arr);
?>

💡 Tip: $arr[] = $val is PHP's most common append syntax — it's more concise than array_push() and marginally faster (no function call overhead).

5. Traversing Arrays

PHP

<?php
$fruits = ["apple", "banana", "orange"];

// foreach for indexed arrays (value only)
foreach ($fruits as $fruit) {
    echo $fruit . "<br>";
}

// foreach for associative arrays (key and value)
$user = ["name" => "John", "age" => 25, "city" => "New York"];
foreach ($user as $key => $value) {
    echo "{$key}: {$value}<br>";
}
// name: John
// age: 25
// city: New York
?>

foreach doesn't care whether the array is indexed or associative — it traverses everything gracefully. That's the beauty of PHP's unified array type.

▶ サンプル: Traversing Nested Arrays

PHP

<?php
// Real-world: iterating through a student grade sheet
// (mixing associative and indexed arrays)
$classroom = [
    ["name" => "Alice", "scores" => ["Math" => 85, "English" => 92]],
    ["name" => "Bob", "scores" => ["Math" => 78, "English" => 88]],
];
foreach ($classroom as $student) {
    $total = array_sum($student["scores"]);
    echo "{$student['name']} — Total: {$total}<br>";
}
?>

▶ 試してみよう

6. Debugging Arrays

PHP

<?php
$user = [
    "name" => "John",
    "age" => 25,
    "skills" => ["PHP", "MySQL", "JavaScript"]
];

// var_dump: the most detailed debug output (includes type and length)
var_dump($user);
/*
array(3) {
  ["name"]=> string(4) "John"
  ["age"]=> int(25)
  ["skills"]=> array(3) { [0]=> ... }
}
*/

// print_r: a concise version (shows structure only, no types)
print_r($user);
/*
Array ( [name] => John [age] => 25 [skills] => Array (...) )
*/

// In HTML, use <pre> tags for cleaner output
echo "<pre>";
print_r($user);
echo "</pre>";
?>

💡 Tip: Need to see the full array structure during development? → var_dump(). Just need a quick look at the content? → echo "<pre>" + print_r(). In production? → Never output arrays directly; use logging.

7. count() — Counting Elements

PHP

<?php
$fruits = ["apple", "banana", "orange"];
echo count($fruits);     // 3

$empty = [];
echo count($empty);      // 0

// sizeof is an alias for count — they're identical
echo sizeof($fruits);    // 3
?>

8. Checking If a Key or Value Exists

PHP

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

// Check if a key exists
var_dump(array_key_exists("name", $user));   // bool(true)
var_dump(array_key_exists("email", $user));  // bool(false)

// Check if a value exists
var_dump(in_array("John", $user));   // bool(true)
var_dump(in_array("Jane", $user));   // bool(false)
?>

▶ サンプル: Safe Array Access Patterns

PHP

<?php
// When you're not sure a key exists, check first
$user = ["name" => "John"];

// Safe pattern
if (isset($user["age"])) {
    echo $user["age"];
} else {
    echo "Age not set";
}

// Concise pattern: use ?? to provide a default
$age = $user["age"] ?? "Unknown";
echo $age;  // Unknown (because the "age" key doesn't exist)
?>

▶ 試してみよう

💡 Tip: isset() vs. ??: isset($arr['key']) checks that the key exists and the value is not null. array_key_exists('key', $arr) checks only whether the key exists (returns true even if the value is null).

❓ よくある質問

Q How is a PHP array different from a JavaScript array?

A A PHP array is an all-in-one structure (list + dictionary rolled into one). JavaScript's Array is just a list; Object/Map handles the dictionary role. In PHP, [0=>"a", 1=>"b"] and ["x"=>1, "y"=>2] are the exact same type.

Q What's the difference between indexed and associative arrays?

A To PHP, there's essentially no difference — both are ordered maps. Indexed arrays have auto-assigned integer keys starting from 0. Associative arrays have custom string keys. You can even mix them: ["a"=>1, 2=>3, 4=>5] is completely valid.

Q Why doesn't my foreach modify the original array?

A Unless you use &$val for reference traversal, foreach ($arr as $val) gives $val as a copy of each array element. Modifying $val doesn't affect the original array.

📖 まとめ

PHP arrays are ordered maps — indexed and associative arrays are the same type under the hood
Use [] to create arrays; $arr[key] to access / modify elements
$arr[] = $val appends an element; unset($arr[key]) removes one
foreach ($arr as $key => $val) traverses arrays
var_dump() / print_r() debug array structures
isset() / ?? / array_key_exists() enable safe access
count() counts elements; in_array() checks values; array_key_exists() checks keys

📝 練習問題

Create an indexed array with 5 city names, output a numbered list with foreach, then use $arr[] to append one more city and re-output the list.
Create an associative user array with at least 3 properties (name, age, email, skills list), then use foreach to display it as an HTML table. Handle the skills list (itself an array) specially — output it as a comma-separated string.
Write a function safeGet($arr, $key, $default) that safely retrieves a value from an array by key, returning the default value if the key doesn't exist.

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