First PHP program (Hello World) - Time & Space Complexity

Time Complexity: First PHP program (Hello World)

O(1)

Understanding Time Complexity

We want to see how the time to run a simple PHP program changes as the input size changes.

How does the program's work grow when we run it?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

<?php
  echo "Hello World!";
?>

This code prints the message "Hello World!" once.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Printing a single message.
• How many times: Exactly once.

How Execution Grows With Input

Since the program prints only one message, the work stays the same no matter what.

Input Size (n)	Approx. Operations
10	1
100	1
1000	1

Pattern observation: The work does not grow with input size; it stays constant.

Final Time Complexity

Time Complexity: O(1)

This means the program takes the same amount of time no matter how big the input is.

Common Mistake

[X] Wrong: "The program takes longer if I run it many times or with bigger input."

[OK] Correct: This program runs the print command only once, so its time does not depend on input size.

Interview Connect

Understanding simple programs helps build a strong base for more complex coding tasks.

Self-Check

"What if we changed the program to print the message inside a loop that runs n times? How would the time complexity change?"

Practice

1. What does the echo statement do in a PHP program?

easy

A. It prints text or output to the screen.

B. It starts the PHP program.

C. It ends the PHP program.

D. It declares a variable.

Solution

1. Step 1: Understand the role of echo

   The echo statement is used in PHP to display text or output on the screen.

2. Step 2: Differentiate from other PHP parts

   Starting and ending PHP code uses <?php and ?>, not echo. Variables are declared differently.

3. Final Answer:

   It prints text or output to the screen. -> Option A

4. Quick Check:

   echo = print output [OK]

Hint: Remember echo means print output on screen [OK]

Common Mistakes:

• Confusing echo with PHP start tag
• Thinking echo ends the program
• Mixing echo with variable declaration

2. Which of the following is the correct way to start a PHP script?

easy

A. <script>

B. <php>

C. <?>

D. <?php

Solution

1. Step 1: Identify the PHP opening tag

   PHP scripts start with <?php to tell the server to process PHP code.

2. Step 2: Check other options

   <php> and <?> are invalid tags. <script> is for JavaScript, not PHP.

3. Final Answer:

   <?php -> Option D

4. Quick Check:

   PHP starts with <?php [OK]

Hint: PHP always starts with <?php tag [OK]

Common Mistakes:

• Using <php> instead of <?php
• Confusing PHP tags with HTML or JavaScript tags
• Omitting the PHP start tag

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

<?php
echo "Hello World!";
?>

medium

A. echo "Hello World!";

B. Hello World!

C. Syntax Error

D. No output

Solution

1. Step 1: Understand the echo statement

   The echo command prints the string inside quotes to the screen exactly as it is.

2. Step 2: Check the code syntax

   The code uses correct PHP tags and syntax, so it will output the string "Hello World!".

3. Final Answer:

   Hello World! -> Option B

4. Quick Check:

   echo prints "Hello World!" [OK]

Hint: echo prints text inside quotes directly [OK]

Common Mistakes:

• Expecting code to print the echo statement itself
• Missing semicolon causing syntax error
• Not using correct PHP tags

4. Find the error in this PHP code:

<?php
echo "Hello World!"
?>

medium

A. Incorrect quotes around text

B. Wrong PHP start tag

C. Missing semicolon after echo statement

D. Missing closing PHP tag

Solution

1. Step 1: Check the echo statement syntax

   In PHP, each statement must end with a semicolon. The echo line is missing it.

2. Step 2: Verify other parts

   The PHP start tag and quotes are correct. The closing PHP tag is optional but present.

3. Final Answer:

   Missing semicolon after echo statement -> Option C

4. Quick Check:

   Statements need semicolon [OK]

Hint: Always end PHP statements with a semicolon [OK]

Common Mistakes:

• Forgetting semicolon at end of statement
• Confusing PHP tags with HTML tags
• Using wrong quotes like single without closing

5. You want to write a PHP program that prints "Hello World!" and then prints "Welcome to PHP." on the next line. Which code is correct?

hard

A. <?php
echo "Hello World!";
echo "Welcome to PHP.";
?>

B. <?php
echo "Hello World!\nWelcome to PHP.";
?>

C. <?php
print "Hello World!" print "Welcome to PHP.";
?>

D. <?php
echo "Hello World!" echo "Welcome to PHP.";
?>

Solution

1. Step 1: Check syntax for multiple echo statements

   Each echo must end with a semicolon.

   <?php
   echo "Hello World!";
   echo "Welcome to PHP.";
   ?>

   correctly uses two echo statements with semicolons.

2. Step 2: Check other options for errors

   <?php
   echo "Hello World!\nWelcome to PHP.";
   ?>

   uses \n inside double quotes but PHP does not automatically print new lines in HTML output.

   <?php
   print "Hello World!" print "Welcome to PHP.";
   ?>

   misses semicolon between print statements.

   <?php
   echo "Hello World!" echo "Welcome to PHP.";
   ?>

   misses semicolon after first echo.

3. Final Answer:

   Two separate echo statements with semicolons -> Option A

4. Quick Check:

   Separate echo with semicolons for multiple lines [OK]

Hint: Use separate echo lines with semicolons for multiple outputs [OK]

Common Mistakes:

• Missing semicolons between statements
• Using print without semicolons
• Expecting \n to create new line in browser output