PHP Associative Arrays

Before you start

PHP arrays.

Map names to values and loop over keys and values.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A PHP array is an ordered map. With string keys it works as a dictionary: ["Ada" => 91] pairs the key with a value, and $scores["Linus"] = 79 adds one more entry at the end.

foreach ($scores as $name => $score) gives the loop both the key and the value. Variables inside a double-quoted string are replaced with their values, and count returns the number of entries.

Code example

PHP

<?php
$scores = ["Ada" => 91, "Grace" => 88];
$scores["Linus"] = 79;
foreach ($scores as $name => $score) {
    echo "$name: $score\n";
}
echo count($scores), "\n";

Output

Ada: 91
Grace: 88
Linus: 79
3

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

Guided walkthroughReady

1<?php

2$scores = ["Ada" => 91, "Grace" => 88];

3$scores["Linus"] = 79;

4foreach ($scores as $name => $score) {

5 echo "$name: $score\n";

6}

7echo count($scores), "\n";

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Two key-value pairs.	`scores = ["Ada" => 91, "Grace" => 88]`	—
A new key is appended.	`3 entries`	—
Each pass prints one pair, in insertion order.	`3 entries`	Ada: 91 Grace: 88 Linus: 79
Print the number of entries.	`3 entries`	Ada: 91 Grace: 88 Linus: 79 3

Behind the scenes

How PHP processes this code

The PHP runtime executes the script. echo writes to the response or terminal output.

Cost and common mistakes

Lookup and insert are O(1) on average; the loop is O(n).

Reading a missing key raises a warning and gives null; test with isset or array_key_exists first.
Numeric-looking string keys such as "5" are converted to integer keys.

Try it yourself

Remove Grace before the loop.

Reveal answer and reasoning

Call unset($scores["Grace"]); the loop then prints two lines and count gives 2.