Comments in PHP - Step-by-Step Execution

Concept Flow - Comments in PHP

Start PHP code

↓

Read line

↓

Is line a comment?

Yes→Ignore line, no output

No↓

Execute line

↓

More lines?

Yes→Read next line

No↓

End PHP code

PHP reads each line, ignores comment lines, and executes only the code lines.

Execution Sample

PHP

<?php
// This is a single-line comment
# Another single-line comment
/* This is a
   multi-line comment */
echo "Hello"; // Prints Hello
?>

This code shows different PHP comment styles and prints 'Hello'.

Execution Table

Step	Line Content	Is Comment?	Action	Output
1	<?php	No	Start PHP code
2	// This is a single-line comment	Yes	Ignore line
3	# Another single-line comment	Yes	Ignore line
4	/* This is a	Yes	Ignore line
5	multi-line comment */	Yes	Ignore line
6	echo "Hello"; // Prints Hello	No	Execute echo statement	Hello
7	?>	No	End PHP code

💡 Reached end of PHP code, all comments ignored, only echo outputs 'Hello'

Variable Tracker

Variable	Start	After Step 6	Final
Output	""	"Hello"	"Hello"

Key Moments - 3 Insights

Why does the line with // not produce any output?

Because lines starting with // are comments, PHP ignores them as shown in steps 2 and 6 in the execution_table.

Does the multi-line comment affect the output?

No, the multi-line comment between steps 4 and 5 is ignored by PHP, so it does not affect the output.

What happens to the code after the comment on the echo line?

PHP executes the code before the comment (echo "Hello"), and ignores the comment after //, so it prints 'Hello' only.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after step 6?

A "// This is a single-line comment"

B "Hello"

C ""

D "# Another single-line comment"

💡 Hint

Check the Output column at step 6 in the execution_table.

At which step does PHP stop ignoring comment lines and produce output?

A Step 6

B Step 4

C Step 2

D Step 7

💡 Hint

Look at the Action and Output columns in the execution_table.

If the multi-line comment was removed, how would the output change?

A Output would be empty

B Output would include the comment text

C Output would be "Hello" as before

D Code would cause an error

💡 Hint

Refer to the key_moments about multi-line comments and the execution_table rows 4 and 5.

Concept Snapshot

PHP comments start with // or # for single-line, and /* */ for multi-line.
Comments are ignored by PHP and do not affect output.
Code after comments runs normally.
Use comments to explain code without changing behavior.

Full Transcript

This visual trace shows how PHP reads code line by line. Lines starting with //, #, or enclosed in /* */ are comments and ignored. Only code lines run and produce output. For example, echo "Hello" prints Hello, even if followed by a comment. Comments help explain code without affecting what PHP does.

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