PHP lesson 23: Sorting: bubble sort and sort()
US version. US grades and US spelling. · UK version of this lesson
A bubble sort goes through the array comparing each item with the NEXT one and swapping them if they are the wrong way round. After the first pass, the biggest item has "bubbled" to the end. Repeat until a pass makes no swaps. A complete 40-minute PHP lesson (Grades 9–12), from Unit 8: Algorithms and a project. It runs right here in your browser – nothing to install.
- Time
- 40 minutes + 20 min homework
- Grade
- Grades 9–12 (ages 14–18)
- You need
- A computer or tablet, paper and a pencil
- Graded
- Instantly, with explanations
For parents: how to use this lesson
You don't need to know PHP yourself. The lesson explains each step on screen, the way Mr Babbage, our Head of Computing, likes it: a worked example first, then a go for your child. Your child predicts what code prints, traces it and puts it in order – graded straight away – and then writes and fixes real PHP in the practice test, where it runs in the browser.
- Read what your child will learn together.
- Press Start the lesson below. Sit with your child for the teaching and the worked example (about 16 minutes): ask them to say what each line does.
- Let them do the practice on their own, then the write-it and fix-it questions in the practice test. When code goes wrong, PHP's message is a clue – let them read it out and try again before you help.
- Finish with the talk question: How many passes can a bubble sort of 5 items need at most? Why?
Watch out for
- One pass sorts the array. One pass only moves the biggest to the end.
sort()returns the sorted array. It sorts in place and returns true.
Miss Lovelace's tip for home: ask your child to explain one line of their program to you in their own words. If they can explain it, they understand it – and every message from PHP is a clue, not a failure.
What your child will learn
Learning goal: trace a bubble sort, pass by pass.
Success looks like
- I can trace a bubble sort, pass by pass.
- I can write the swap of two items.
- I can use PHP's own
sort()andrsort()and know what they return.
Key words
sorting · bubble sort · pass · swap · comparison · sort()
What to have ready
- A device with a web browser (PHP runs in the browser – nothing to install)
- Paper for tracing
- A keyboard helps for typing code (a tablet or phone works too)
The big ideas (for the adult)
A bubble sort goes through the array comparing each item with the NEXT one and swapping them if they are the wrong way round. After the first pass, the biggest item has "bubbled" to the end. Repeat until a pass makes no swaps.
To swap two items you need a spare variable: $t = $a[$j]; $a[$j] = $a[$j + 1]; $a[$j + 1] = $t;.
Real programs usually call PHP's sort() – it is faster and already tested. It changes the array itself and returns only true, so $a = sort($a); is a bug that throws your data away.
Worked example – see it
<?php
$a = ["museum", "school", "guitar", "dragon"];
sort($a);
echo implode(" ", $a);It prints:
dragon guitar museum school
The lesson, step by step
| Part | Time | How it is checked |
|---|---|---|
| Starter | 5 min | Graded by Classfern |
| Teaching input | 8 min | Read, watch and talk |
| See it, then predict | 8 min | Graded by Classfern |
| Put it in order, trace it, check it | 14 min | Graded by Classfern |
| Closure and exit ticket | 5 min | Graded by Classfern + self-check |
The lesson
It runs here on the page. No sign-up and no names – progress is kept only in this browser. Each visit picks a new mix of questions.
Homework
20 minutes: 5 questions – predict what code prints, trace it or put it in order, and check the key ideas – then a short reflection. New questions every time, all graded instantly.
Write it and fix it: practice tests
The practice tests run real PHP in your browser: write and fix short programs, graded against hidden tests.
Curriculum
CSTA K–12 Computer Science Standards (2017): 3B-AP-10 – Use and adapt classic algorithms to solve problems; 3B-AP-11 – Evaluate algorithms for efficiency, correctness and clarity.
The standard codes are CSTA's; the descriptions are our own words.
Our lessons and questions are our own. They are not written or endorsed by any government department, testing agency or exam board. Classfern is not affiliated with or endorsed by The PHP Group. PHP runs here as php-wasm, under its open-source licences.