In this lesson, you’ll learn about the do-while loop in PHP. You’ll see what it is, when to use it, and how to write clear and safe code. We’ll show practical PHP code examples with comments, discuss best practices, and cover common mistakes. This is the perfect continuation after the lessons on the while loop and for loop.

Why is the do-while loop important?

The do-while loop in PHP is a control structure that executes a block of code at least once, and then repeats it as long as the condition remains true. It is particularly useful when:

you want to display a menu and require user choice,
you need to perform input validation (e.g., email address) at least once,
you implement retries of an operation (e.g., API connection),
you want to repeat an action until a condition is met (e.g., rolling a dice until a six).

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Basics: how does do-while work?

Syntax

<?php
do {
    // loop body — always executes at least once
} while ($condition); // NOTE: semicolon at the end is required!

First, the block inside do { ... } executes.
Then PHP checks the ($condition).
If true, the loop repeats; if false, the loop ends.

How is do-while different from while and for?

while: checks condition first, may never run.
do-while: runs body first, then checks condition, so it always runs at least once.
for: best for loops with a counter and known number of iterations.

When to choose do-while? When you need to guarantee at least one execution — like showing a prompt or menu.

PHP Code Examples

1) Simple counter 1..5

<?php
$i = 1;

do {
    echo "Iteration: $i\n";
    $i++;
} while ($i <= 5);

2) Email validation in CLI

<?php
do {
    $email = readline("Enter a valid email (or 'q' to quit): ");

    if ($email === 'q') {
        echo "Aborted.\n";
        break;
    }

    $valid = filter_var($email, FILTER_VALIDATE_EMAIL);

    if (!$valid) {
        echo "That doesn’t look like a valid email. Try again.\n";
    }
} while (!$valid);

if (!empty($valid)) {
    echo "Thanks! Your email: $email\n";
}

3) Simple menu

<?php
do {
    echo "\n=== MENU ===\n";
    echo "1) Show time\n";
    echo "2) Roll dice\n";
    echo "0) Exit\n";

    $choice = readline("Choose option: ");

    switch ($choice) {
        case '1':
            echo "Time: " . date('Y-m-d H:i:s') . "\n";
            break;
        case '2':
            echo "Rolled: " . random_int(1, 6) . "\n";
            continue;
        case '0':
            echo "Goodbye.\n";
            break 2;
        default:
            echo "Unknown option. Try again.\n";
    }
} while (true);

4) Rolling until a six

<?php
$tries = 0;

do {
    $tries++;
    $roll = random_int(1, 6);
    echo "Try $tries: $roll\n";
} while ($roll !== 6);

echo "Success after $tries tries!\n";

5) Iterating over an array with safety check

<?php
$users = ['Anna', 'Bob', 'Chris'];
$index = 0;

if (count($users) > 0) {
    do {
        echo "User: {$users[$index]}\n";
        $index++;
    } while ($index < count($users));
} else {
    echo "No users found.\n";
}

6) Retry pattern with attempt limit

<?php
function fragileOperation(): bool {
    return random_int(0, 1) === 1;
}

$maxAttempts = 5;
$attempt = 0;
$success = false;

do {
    $attempt++;
    echo "Attempt $attempt...\n";
    try {
        $success = fragileOperation();
    } catch (Throwable $e) {
        $success = false;
    }

    if (!$success && $attempt < $maxAttempts) {
        echo "Retrying...\n";
        usleep(200_000);
    }
} while (!$success && $attempt < $maxAttempts);

if ($success) {
    echo "Success on attempt $attempt.\n";
} else {
    echo "Failed after $maxAttempts attempts.\n";
}

Best Practices and Common Mistakes

Best Practices

Always update loop state inside (e.g., counters).
Use do-while when logic requires at least one execution.
Add fallback exits (attempt limits, breaks) to avoid infinite loops.
Keep conditions simple and use strict comparisons (===).
Prefer foreach for arrays, do-while for prompts, retries, menus.

Common Mistakes

Missing semicolon after while ($cond).
Assignment instead of comparison in condition.
Forgetting to update the counter → infinite loop.
Accessing arrays without checking for emptiness.
Using do-while unnecessarily when while/for is clearer.

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Summary

do-while executes block at least once, then repeats while condition is true.
Ideal for prompts, menus, retries, actions until condition.
Remember semicolon, counter updates, strict conditions, and escape exits.
Use the right loop type for the right task.

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Mini Quiz

What’s the difference between while and do-while?

➡️ do-while runs body at least once, while may not run at all.

Which syntax is correct?

➡️ do { ... } while ($x > 0);

How to safely break from infinite do-while?

➡️ Use break with condition or attempt limit.

When is do-while better than while?

➡️ When code must run at least once (prompt, menu).

What’s wrong here?

$i = 0;
do {
    echo $i;
} while ($i < 5);

➡️ \$i never changes → infinite loop.

True/False: The semicolon after while condition is optional.

➡️ False – it’s required.

Now you know how to use the do-while loop in PHP to write safe, readable code for menus, prompts, retries, and repeat-until tasks.