Single-line comments (//) are suitable for short, local instructions or debugging, 1. Use // for in-line comments or temporarily disable code; 2. Use // for multi-line comments to provide detailed descriptions of complex logic or comment large pieces of code; 3. Use /*/ to write PHPDoc to implement structured documents and integrate with the IDE; 4. Avoid comments to comments to be obvious; 5. Always keep comments updated to ensure comments clearly convey intent rather than just describe operations, thereby improving code maintainability.

When it comes to writing clean, maintainable PHP code, choosing between multiline ( /* */ ) and single-line ( // or # ) comments might seem trivial—but the right choice can significantly impact readability, debugging, and team collaboration. While both have their place, understanding when and why to use each is key to writing professional-grade PHP.

When to Use Single-Line Comments

Single-line comments ( // or # ) are ideal for quick, localized explanations. They're lightweight and don't require closing syntax, making them perfect for inline notes or temporarily disabling code.

Use them when:

• You're explaining a specific line or short block of logic.
• You're debugging and want to comment out a few lines quickly.
• You're working within a section where brevity is more important than formality.

 // Calculate user discount based on membership tier
$discount = $user->isPremium() ? 0.2: 0.05;

# Legacy fallback – remove after Q3 migration
// $oldRate = getLegacyRate();

Tip: Stick with // over # —it's more widely used in PHP and consistent with C-style languages, improving cross-language readingability.

When Multiline Comments Are the Better Choice

Multiline comments ( /* ... */ ) shine when you need to provide detailed context, document complex logic, or wrap larger blocks of code. They're especially useful for formal documentation or commenting out entire functions.

Use them when:

• Writing function or class-level documentation (though PHPDoc is better—more on that below).
• Temporarily disabling a large block of code during development.
• Including copyright notices, changelogs, or licensing info at the top of a file.

 /*
 * This function calculates compound interest over time
 * Parameters:
 * - $principal: initial amount
 * - $rate: annual interest rate (decimal)
 * - $timesCompounded: yearly frequency
 * - $years: duration
 */
function calculateCompoundInterest($principal, $rate, $timesCompounded, $years) {
    // ...
}

Note: Unlike // , /* */ comments can span multiple lines and even be nested in theory —but avoid nesting, as it can break syntax highlighting and confuse developers.

Don't Confuse Comments with PHPDoc

It's important to distinguish regular comments from PHPDoc blocks . For documenting classes, methods, parameters, and return types, use PHPDoc syntax with /** */ —not plain multiline comments.

? Correct:

 /**
 * Validates user input and returns sanitized data.
 *
 * @param array $input Raw user data
 * @return array Sanitized and validated data
 * @throws ValidationException If input is invalid
 */
function validateUserData(array $input): array
{
    // ...
}

? Not ideal:

 /* 
 * Validates user input...
 */

PHPDoc is structured, tool-friendly, and integrates with IDEs and static analyzers. Regular comments don't offer that.

Best Practices Summary

To make smarter commenting decisions:

• Use // for short, line-specific notes or debugging.
• Use /* */ for multi-line explanations or disabling code blocks.
• Always use /** */ for PHPDoc—never substitute it with regular comments.
• Avoid over-commenting obvious code (eg, // set $x to 5 ).
• Keep comments updated—outdated comments are worse than no comments.

Ultimately, the goal isn't just to comment, but to clarify intent. Whether single-line or multiline, your comments should help the next developer (or future you) understand why something was done, not just what it does.

Basically: match the comment style to the scope and purpose. Keep it clean, keep it useful.

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