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Control flow interview questions

11 interview questions on control flow from PHP (8.5): short warm-ups, then what each harder question tests, the first step of a good answer and its follow-ups.

11 questions: 3 warm-ups and 8 with model answers · Levels: Fresher to senior

Warm-up questions

Short questions with short answers, each answered in full.

Fresher

Are elseif and else if the same in PHP?

Answered in the lesson if, else, elseif and the ternary operator
Fresher

How do you loop over the keys and values of an array in PHP, and does changing the loop variable change the array?

Answered in the lesson Loops: for, while, do-while and foreach
Fresher

What is the difference between include and require, and when do you use their _once forms?

Answered in the lesson include and require: splitting code into files

Questions with model answers

What each question tests, the first step of a good answer and the follow-ups to expect.

Fresher to mid

A checkout function prices delivery with if statements nested five levels deep: member or not, coupon or not, metro or not, order size, weekend or not. Members sometimes pay the non-member price. How do you find the cause, and how do you restructure the function so that the next mistake is easy to see?
What it tests

Reading a long chain of conditions, the order of tests, guard clauses, and boundary tests that prove a refactor changed nothing.

First step

Write the rules down as a table of inputs and fees, with one row just below and one exactly on each threshold, for members and non-members. Run the current function on every row before changing anything: the rows that come out wrong point to the branch that is tested too late or never reached.
Follow-ups
- Which conditions become guard clauses, and in what order?
- How do you prove the restructured function returns the same fees?
- When would a match (true) be clearer than the chain?
From the lesson if, else, elseif and the ternary operator
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In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to mid

Reading a signup handler, you find $valid = $hasEmail and $hasPhone;, and its tests pass. What does the line really store, why did the tests miss it, and how do you find and fix similar lines in a large codebase?
What it tests

The precedence of and and or against =, test inputs that make a condition false, and finding a risky pattern across a codebase.

First step

Explain the grouping first: = binds before and, so PHP runs ($valid = $hasEmail) and $hasPhone, and $valid ends up equal to $hasEmail. Then look at the test inputs: if every test that reaches the line has both an email and a phone, or neither, the bug cannot show.
Follow-ups
- Is $handle = fopen($path, 'r') or die('…'); wrong too?
- Which test inputs would have caught it?
- How do you keep it from coming back?
From the lesson if, else, elseif and the ternary operator
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to mid

Write gradeReport(array $marks): array for an array of names to marks. It returns the count of each grade (A from 85, B from 70, C from 50, F below 50) and the names with the highest mark. How do you do it in one loop, and what happens with ties and an empty class?
What it tests

foreach over keys and values, counters that start at zero, tracking a maximum with ties, and the empty input.

First step

Start the four counters at 0 and the list of top names empty, then make one pass with foreach ($marks as $name => $mark): add one to the counter of the mark's grade, and compare the mark with the best mark seen so far.
Follow-ups
- How do you keep every name when two students share the top mark?
- What should the function return for an empty class?
- Would array functions be clearer than the loop?
From the lesson Loops: for, while, do-while and foreach
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to mid

An export script trims every field of $rows in a foreach by reference, then a second foreach writes the CSV file. The file has the right number of rows, but its last row is a copy of the second-to-last. Explain why, give the one-line fix, and write a version without references.
What it tests

The reference that foreach leaves behind, how a later loop writes through it, and choosing a fix.

First step

Look at the variable names of the two loops. If both use $row and the first takes &$row, then after the first loop $row is still a reference to the last element, and every value the second loop assigns to $row is written into that element.
Follow-ups
- Why does the file still have the right number of rows?
- Where exactly does the fix belong?
- How do you test for it?
From the lesson Loops: for, while, do-while and foreach
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Mid to senior

A nightly PHP command-line job pages through an API and is sometimes still running in the morning, until someone kills it. Why does PHP not stop it, which loop bugs cause this, and how do you make the job end safely every night?
What it tests

The time limit on the command line, conditions that never become false, guards on loops over outside data, and failing in a way someone notices.

First step

Check the time limit first: on the command line PHP sets none by default, so it will not stop the job by itself. Then read the exit condition of the paging loop and ask what happens if the cursor stops changing or never becomes null.
Follow-ups
- Would set_time_limit() alone fix it?
- Which guards does the loop need?
- How would you know before the morning?
From the lesson Loops: for, while, do-while and foreach
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to mid

A report script works when you run it from its own folder, but its cron job fails every night with "Failed opening required '../config/app.php'". Why, and how do you write include paths that work wherever the script is started from?
What it tests

The working directory against the file's own folder, how relative paths and bare names are resolved, and __DIR__.

First step

Compare the two working directories. From a terminal it is the folder you typed the command in, while cron starts the job from whatever folder its own settings give. A path that starts with ../ is resolved against that folder, not against the folder of the script.
Follow-ups
- Why does the same path work from a terminal?
- Is chdir(__DIR__) at the top of the script a good fix?
- Where does __DIR__ point inside an included file?
From the lesson include and require: splitting code into files
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Mid to senior

A legacy site loads every page with include $_GET['page'] . '.php';. Explain the risk to the team that owns the code, fix it without breaking the existing links, and describe the test that proves the fix.
What it tests

Recognising local file inclusion, an allowlist that keeps old links working, and a test that fails on the weak version and passes on the fix.

First step

Say who decides what runs: the address does, so any name a visitor writes picks a file on the server, and a name with ../ in it reaches files outside the pages. Then collect the page names the site really uses, from its links and its access logs, because those are the names the fix must keep.
Follow-ups
- Why is checking that the file exists not enough?
- What about names that must come from data, such as help articles?
- Which test belongs in the suite?
From the lesson include and require: splitting code into files
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to mid

A layout template shows an internal error message to customers on some pages. The controller set $message earlier for its log, and the template prints $message whenever it is set. Why can the template see it, and how do you limit what each template can read?
What it tests

The shared scope of include, a function as a scope boundary, extract() and its flags, and where escaping belongs.

First step

State the rule first: an included file inherits the variables of the line that includes it, so a template included at the top level of a controller sees every variable the controller has set, including ones meant only for the log.
Follow-ups
- Would renaming the variable be enough?
- How does a render() function help, and what can still leak?
- Where does escaping belong?
From the lesson include and require: splitting code into files
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes

All PHP (8.5) interview questions · The PHP (8.5) track

Warm-up questions

Are elseif and else if the same in PHP?

How do you loop over the keys and values of an array in PHP, and does changing the loop variable change the array?

What is the difference between include and require, and when do you use their _once forms?

Questions with model answers

Reading a signup handler, you find $valid = $hasEmail and $hasPhone;, and its tests pass. What does the line really store, why did the tests miss it, and how do you find and fix similar lines in a large codebase?

Write gradeReport(array $marks): array for an array of names to marks. It returns the count of each grade (A from 85, B from 70, C from 50, F below 50) and the names with the highest mark. How do you do it in one loop, and what happens with ties and an empty class?

An export script trims every field of $rows in a foreach by reference, then a second foreach writes the CSV file. The file has the right number of rows, but its last row is a copy of the second-to-last. Explain why, give the one-line fix, and write a version without references.

A nightly PHP command-line job pages through an API and is sometimes still running in the morning, until someone kills it. Why does PHP not stop it, which loop bugs cause this, and how do you make the job end safely every night?

A report script works when you run it from its own folder, but its cron job fails every night with "Failed opening required '../config/app.php'". Why, and how do you write include paths that work wherever the script is started from?

A legacy site loads every page with include $_GET['page'] . '.php';. Explain the risk to the team that owns the code, fix it without breaking the existing links, and describe the test that proves the fix.

A layout template shows an internal error message to customers on some pages. The controller set $message earlier for its log, and the template prints $message whenever it is set. Why can the template see it, and how do you limit what each template can read?