Arrays

Course 1 · Ch 6

Arrays: Indexed, Associative, and Common Array Functions

Storing collections of values — by position or by name — and the built-in functions for working with them

Arrays appeared briefly in Chapter 4's foreach examples. This chapter covers them properly: the two main styles PHP arrays come in, and the handful of built-in functions used constantly in real PHP code.

Indexed Arrays — Position-Based

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

  echo $fruits[0];   // "apple" — indexing starts at 0, not 1
  echo $fruits[2];   // "cherry"

  $fruits[] = "date";       // appends to the end — now index 3
  print_r($fruits);          // dumps the whole array readably
?>

PHP arrays are zero-indexed — the first element is index 0, not 1

This trips up plenty of beginners coming from everyday counting (where you'd naturally call "apple" the "first" item and expect it at position 1). $fruits[0] is the first element; $fruits[count($fruits) - 1] is the last.

Associative Arrays — Named Keys

<?php
  $person = [
    "name" => "Philip",
    "age" => 35,
    "city" => "London"
  ];

  echo $person["name"];   // "Philip"
  $person["email"] = "philip@example.com";  // adds a new key
?>

An associative array uses meaningful string keys instead of numeric positions — genuinely more readable than remembering "index 1 is always the age" when modelling real-world data like a person's details.

Multidimensional Arrays — Arrays of Arrays

<?php
  $people = [
    ["name" => "Philip", "age" => 35],
    ["name" => "Sam",    "age" => 28]
  ];

  echo $people[0]["name"];   // "Philip"

  foreach ($people as $person) {
    echo $person["name"] . " is " . $person["age"] . "<br>";
  }
?>

An array's elements can themselves be arrays — extremely common for representing lists of records, like rows that might later come from a database (covered in a future course on database integration).

Common Built-In Array Functions

Function	What it does
count($arr)	Number of elements
array_push($arr, $val)	Adds to the end (same as $arr[] = $val)
array_pop($arr)	Removes and returns the last element
array_merge($a, $b)	Combines two arrays into one
in_array($val, $arr)	Checks whether a value exists in the array
array_search($val, $arr)	Returns the key/index of a value, or false if not found
sort($arr)	Sorts values, re-indexes keys from 0
array_map($fn, $arr)	Applies a function to every element, returns a new array
array_filter($fn, $arr)	Keeps only elements where the function returns true

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

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

  // array_filter — keep only matching elements
  $evens = array_filter($numbers, function($n) { return $n % 2 == 0; });
  print_r($evens);     // [1 => 2, 3 => 4] — note original keys are preserved!
?>

array_map and array_filter take an anonymous function as an argument

The function($n) { ... } passed in is a function with no name, defined right where it's used — this pattern (a "callback") is genuinely common in PHP once you start using these array functions. It will feel more natural with practice; for now, treat it as "the rule to apply to each element," written inline rather than as a separately-named function.

array_filter preserves original keys — this surprises people expecting a clean 0,1,2... result

After filtering, the remaining elements keep whatever index/key they originally had, leaving gaps. If a clean re-indexed array is needed afterward, wrap the result in array_values().

Coding Challenges

Challenge 1

Create an associative array representing a book with keys "title", "author", and "year". Echo a formatted sentence using all three values, then add a new key "genre" and echo the whole array with print_r.

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Challenge 2

Given an indexed array of 6 numbers of your choice, use in_array() to check if 42 is present, array_search() to find the index of a number you know is in there, and sort() to put the array in ascending order. Echo the result of each step.

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Challenge 3

Given an array of 8 numbers, use array_filter() with an anonymous function to keep only numbers greater than 10, then use array_map() on the filtered result to square each remaining number. Wrap the filtered result in array_values() before mapping, and print_r the final array.

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Chapter 6 Quick Reference

Indexed arrays — zero-based numeric positions: $arr[0] is the first element
Associative arrays — named string keys: $arr["key"]
Multidimensional arrays — arrays of arrays, accessed like $arr[0]["key"]
count(), array_push/pop, array_merge, in_array, array_search, sort — everyday array operations
array_map($fn, $arr) — transforms every element, returns a new array
array_filter($fn, $arr) — keeps only matching elements, preserves original keys
array_values($arr) — re-indexes an array from 0, useful after filtering
Next chapter: strings — common string functions, formatting, and manipulation