PHP do-while Loop

last modified April 16, 2025

The PHP do-while loop is a control structure that executes a block of code at least once, then repeats while a condition remains true. Unlike regular while loops, do-while checks the condition after each iteration.

Basic Definitions

The do keyword starts a do-while loop that always executes its block at least once. The while condition is checked after each iteration to determine if the loop should continue.

Syntax: do { code } while (condition);. The condition is any expression that evaluates to boolean true or false. Note the semicolon after the while condition.

Do-while loops are useful when you need to execute code before checking a condition. They're commonly used for input validation and menu systems.

Basic do-while Loop

This example demonstrates a simple do-while loop counting from 1 to 5.

basic_do_while.php

<?php

declare(strict_types=1);

$i = 1;

do {
    echo $i . " ";
    $i++;
} while ($i <= 5);

The code initializes $i to 1. The loop prints the value and increments it. The condition checks if $i is 5 or less. The loop runs exactly 5 times, printing numbers 1 through 5.

Guaranteed First Execution

This example shows how do-while always executes at least once.

guaranteed_execution.php

<?php

declare(strict_types=1);

$count = 10;

do {
    echo "This runs once, even though count is $count";
} while ($count < 5);

The loop executes once despite the condition being false initially. This demonstrates the key difference from regular while loops. The condition is only checked after the first iteration.

User Input Validation

This example uses do-while for validating user input.

input_validation.php

<?php

declare(strict_types=1);

do {
    $input = (int) readline("Enter a number between 1-10: ");
} while ($input < 1 || $input > 10);

echo "Valid input: $input";

The code prompts for input until a valid number between 1-10 is entered. The loop continues while the input is invalid. This pattern ensures the user gets at least one prompt and continues until valid input.

Menu System

This example implements a simple menu system using do-while.

menu_system.php

<?php

declare(strict_types=1);

do {
    echo "1. Option 1\n";
    echo "2. Option 2\n";
    echo "3. Exit\n";
    
    $choice = (int) readline("Select: ");
    
    switch ($choice) {
        case 1: echo "Option 1 selected\n"; break;
        case 2: echo "Option 2 selected\n"; break;
    }
} while ($choice != 3);

echo "Goodbye!";

The loop displays a menu and processes user choices. It continues until the user selects option 3 (Exit). The menu is always shown at least once. Switch statements often pair well with menu systems.

Array Processing

This example processes array elements with do-while.

array_processing.php

<?php

declare(strict_types=1);

$colors = ["red", "green", "blue"];
$index = 0;

do {
    echo strtoupper($colors[$index]) . "\n";
    $index++;
} while ($index < count($colors));

The code processes each element in the $colors array. It converts each color to uppercase and prints it. The loop continues while the index is less than the array length. Note array bounds are checked.

Nested do-while Loops

This example demonstrates nested do-while loops for a multiplication table.

nested_loops.php

<?php

declare(strict_types=1);

$i = 1;

do {
    $j = 1;
    do {
        echo ($i * $j) . "\t";
        $j++;
    } while ($j <= 10);
    
    echo "\n";
    $i++;
} while ($i <= 10);

The outer loop controls rows, while the inner loop handles columns. It prints a 10x10 multiplication table. Each inner loop completes fully before the outer loop advances. Nested loops can create grid patterns.

Break and Continue

This example shows using break and continue in do-while loops.

break_continue.php

<?php

declare(strict_types=1);

$num = 0;

do {
    $num++;
    
    if ($num % 2 == 0) {
        continue; // Skip even numbers
    }
    
    echo "$num ";
    
    if ($num >= 9) {
        break; // Exit loop at 9
    }
} while (true);

echo "\nLoop ended at $num";

The loop prints odd numbers using continue to skip evens. It breaks when reaching 9, despite the always-true condition. Break exits the loop immediately, while continue skips to the next iteration.

Best Practices

Initialization: Initialize variables before the loop.
Termination: Ensure the condition will eventually become false.
Readability: Use clear conditions and proper indentation.
Complexity: Avoid deeply nested do-while structures.
Alternatives: Consider for loops for counter-based iteration.

Source

PHP do-while Documentation

This tutorial covered PHP do-while loops with practical examples showing basic usage, input validation, menu systems, and control flow.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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