Comments in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of comments in PHP?

Comments are used to explain code, make it easier to understand, and to temporarily disable code without deleting it.

beginner

How do you write a single-line comment in PHP?

You can write a single-line comment using either // or # at the start of the line.

beginner

How do you write a multi-line comment in PHP?

Multi-line comments start with /* and end with */. Everything between is ignored by PHP.

beginner

Can comments affect the execution of PHP code?

No, comments are ignored by the PHP engine and do not affect how the code runs.

beginner

Example: What will this PHP code output?

<?php
// echo 'Hello';
echo 'World';
?>

The output will be World because the first line is a comment and does not run.

Which of the following is NOT a valid way to write a comment in PHP?

A <!-- This is a comment -->

B # This is a comment

C // This is a comment

D /* This is a comment */

Option A is an HTML comment, not a PHP comment.

What symbol starts a multi-line comment in PHP?

A //

B <!--

C #

D /*

Multi-line comments in PHP start with /* and end with */.

What happens to the code inside comments when PHP runs?

A It runs normally

B It is ignored

C It causes an error

D It runs twice

Code inside comments is ignored by PHP and does not run.

Which comment style can be used for a single line in PHP?

A <!-- and -->

B /* and */

C // and #

D None of the above

Single-line comments can use // or # in PHP.

Why might a programmer use comments in their PHP code?

A To explain the code to others

B To speed up the code

C To make the code run slower

D To delete code permanently

Comments help explain code to others or yourself later.

Explain the different ways to write comments in PHP and when you might use each.

Think about quick notes versus longer explanations.

Describe how comments affect PHP code execution and why they are important.

Consider what happens when PHP runs code with comments.

Practice

1. Which of the following is a correct way to write a single-line comment in PHP?

easy

A.

B. // This is a single-line comment

C. /* This is a single-line comment */

D. # This is not a comment

Solution

1. Step 1: Identify single-line comment syntax in PHP

   PHP supports single-line comments using // and #.

2. Step 2: Check each option

   // This is a single-line comment uses // correctly. is HTML comment syntax, not PHP. /* This is a single-line comment */ is multi-line comment syntax. # This is a single-line comment is also valid in PHP, so # This is not a comment is correct as well.

3. Final Answer:

   // This is a single-line comment -> Option B

4. Quick Check:

   Single-line comment = // or # [OK]

Hint: Single-line comments start with // or # in PHP [OK]

Common Mistakes:

• Confusing HTML comments with PHP comments
• Using multi-line comment syntax for single-line comments
• Thinking # is not a valid comment

2. Which of the following is the correct syntax for a multi-line comment in PHP?

easy

A. /* This is a multi-line comment */

B. // This is a multi-line comment

C. # This is a multi-line comment

D.

Solution

1. Step 1: Recall multi-line comment syntax in PHP

   PHP uses /* ... */ to write multi-line comments.

2. Step 2: Evaluate each option

   /* This is a multi-line comment */ correctly uses /* ... */. Options B and C are single-line comment styles. is HTML comment syntax.

3. Final Answer:

   /* This is a multi-line comment */ -> Option A

4. Quick Check:

   Multi-line comment = /* ... */ [OK]

Hint: Multi-line comments use /* at start and */ at end [OK]

Common Mistakes:

• Using // or # for multi-line comments
• Confusing HTML comments with PHP comments
• Forgetting to close multi-line comment with */

3. What will be the output of the following PHP code?

<?php
// echo 'Hello';
echo 'World';
?>

medium

A. HelloWorld

B. No output

C. Hello

D. World

Solution

1. Step 1: Understand the effect of the comment

   The line // echo 'Hello'; is commented out, so it does not run.

2. Step 2: Identify the executed code

   The line echo 'World'; runs and prints "World".

3. Final Answer:

   World -> Option D

4. Quick Check:

   Commented out code does not run, so output = World [OK]

Hint: Code after // is ignored, only uncommented code runs [OK]

Common Mistakes:

• Thinking commented code runs
• Confusing output by combining commented and uncommented lines
• Ignoring that echo prints text

4. Find the error in the following PHP code:

<?php
/* This is a comment
 echo 'Hello';
// Missing closing tag
?>

medium

A. PHP tags are incorrect

B. Using // inside multi-line comment is invalid

C. Missing closing */ for multi-line comment

D. echo statement syntax error

Solution

1. Step 1: Check multi-line comment syntax

   The multi-line comment starts with /* but never ends with */.

2. Step 2: Understand consequences of missing closing comment

   Without closing */, PHP treats the rest as comment, causing syntax errors or unexpected behavior.

3. Final Answer:

   Missing closing */ for multi-line comment -> Option C

4. Quick Check:

   Multi-line comments must close with */ [OK]

Hint: Always close multi-line comments with */ [OK]

Common Mistakes:

• Forgetting to close multi-line comments
• Thinking // cannot be inside multi-line comments
• Misreading PHP tags as error

5. You want to temporarily disable a block of code without deleting it. Which comment style should you use in PHP to do this safely for multiple lines?

hard

A. Use /* at start and */ at end of the block

B. Use # at the start of the block

C. // Comment each line with //

D. Wrap the block with

Solution

1. Step 1: Understand commenting multiple lines

   Using /* ... */ allows commenting multiple lines at once safely.

2. Step 2: Evaluate other options

   // Comment each line with // requires adding // on each line, which is tedious. Use # at the start of the block is invalid for multiple lines. Wrap the block with is HTML comment syntax, not PHP.

3. Final Answer:

   Use /* at start and */ at end of the block -> Option A

4. Quick Check:

   Multi-line comments use /* ... */ [OK]

Hint: Use /* ... */ to comment multiple lines quickly [OK]

Common Mistakes:

• Using # or // for many lines without repeating
• Confusing HTML comments with PHP comments
• Not closing multi-line comments properly