String search functions (strpos, strstr) in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of strpos with needle at start?

Consider the following PHP code snippet. What will be the output when searching for the substring 'Hello' in the string 'Hello World' using strpos?

PHP

<?php
$pos = strpos('Hello World', 'Hello');
echo $pos;
?>

A false

B 1

C null

D 0

💡 Hint

Remember that strpos returns the position index starting at 0.

The substring 'Hello' starts at the very beginning of the string, so strpos returns 0, which is a valid position.

❓ Predict Output

intermediate

What does strstr return when substring is found?

What will be the output of this PHP code using strstr to find 'World' in 'Hello World'?

PHP

<?php
$result = strstr('Hello World', 'World');
echo $result;
?>

A World

B Hello World

C orld

D false

💡 Hint

strstr returns the part of the string starting from the first occurrence of the needle.

strstr returns the substring starting from 'World' to the end, which is 'World'.

❓ Predict Output

advanced

What is the output when strpos needle is not found?

What will this PHP code output when searching for 'abc' in 'defgh' using strpos?

PHP

<?php
$pos = strpos('defgh', 'abc');
var_dump($pos);
?>

A 0

B bool(false)

C int(0)

D null

💡 Hint

If the substring is not found, strpos returns false.

Since 'abc' is not in 'defgh', strpos returns boolean false, and var_dump shows bool(false).

❓ Predict Output

advanced

What does strstr return with before_needle=true?

What will this PHP code output when using strstr with the third parameter true to find 'World' in 'Hello World'?

PHP

<?php
$result = strstr('Hello World', 'World', true);
echo $result;
?>

A Hello

B Hello World

C World

D false

💡 Hint

When before_needle is true, strstr returns the part before the needle.

With before_needle=true, strstr returns the substring before 'World', which is 'Hello '.

🧠 Conceptual

expert

Why strpos can return 0 but still be true in condition?

In PHP, why does this code print 'Not Found' even though strpos returns 0?

A Because 0 is treated as false, so it prints 'Found' anyway

B Because the condition treats 0 as false, so it prints 'Not Found', but the code prints 'Found' due to a bug

C Because strpos returns false on failure, and 0 is treated as false, so it prints 'Not Found'

D Because 0 is treated as false, so it prints 'Not Found'

💡 Hint

Check how PHP treats 0 and false in if conditions.

strpos returns 0 when the substring is at the start, but 0 is false in PHP conditions, so the if condition fails and prints 'Not Found'. The code prints 'Found' only if strpos returns a non-false, non-zero value.

Practice

1. What does the PHP function strpos do?

easy

A. Converts a string to uppercase

B. Extracts a substring starting from a specific character

C. Finds the position of a substring inside a string

D. Replaces a substring with another string

Solution

1. Step 1: Understand the purpose of strpos

   The strpos function searches for the first occurrence of a substring inside another string and returns its position.

2. Step 2: Compare with other options

   Options A, B, and C describe different string functions like strtoupper, strstr, and str_replace, which are not what strpos does.

3. Final Answer:

   Finds the position of a substring inside a string -> Option C

4. Quick Check:

   strpos = position finder [OK]

Hint: strpos returns position index of substring [OK]

Common Mistakes:

• Confusing strpos with strstr
• Thinking strpos extracts substring
• Assuming strpos replaces text

2. Which of the following is the correct syntax to find the position of 'cat' in the string $text = 'concatenate' using strpos?

easy

A. strpos('concatenate', 'cat')

B. strpos($text)

C. strpos('cat', $text)

D. strpos($text, 'cat')

Solution

1. Step 1: Identify correct parameter order for strpos

   The first parameter is the full string, the second is the substring to find. So strpos($text, 'cat') is correct.

2. Step 2: Check other options for errors

   strpos('concatenate', 'cat') uses a string literal instead of the variable. strpos('cat', $text) reverses parameters. strpos($text) misses the second parameter.

3. Final Answer:

   strpos($text, 'cat') -> Option D

4. Quick Check:

   strpos(string, substring) [OK]

Hint: First argument is full string, second is substring [OK]

Common Mistakes:

• Swapping the order of parameters
• Omitting the substring argument
• Passing substring as first argument

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

$text = 'hello world';
$pos = strpos($text, 'world');
echo $pos;

medium

A. 6

B. 5

C. 0

D. false

Solution

1. Step 1: Find the position of 'world' in 'hello world'

   The string 'hello world' has 'world' starting at index 6 (counting from 0).

2. Step 2: Understand what strpos returns

   strpos returns the starting position of the substring, so it returns 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Position of 'world' in 'hello world' = 6 [OK]

Hint: Count characters from zero to substring start [OK]

Common Mistakes:

• Counting from 1 instead of 0
• Confusing position with length
• Expecting boolean true instead of position

4. The following code is intended to check if the word 'apple' exists in $fruit. What is the error?

$fruit = 'pineapple';
if (strpos($fruit, 'apple')) {
  echo 'Found';
} else {
  echo 'Not found';
}

medium

A. strpos returns false if found, so condition is wrong

B. strpos returns 0 for 'apple' position, which is treated as false

C. Missing third parameter in strpos

D. Syntax error in if statement

Solution

1. Step 1: Understand strpos return values

   strpos returns 0 if the substring is found at the start (index 0). In PHP, 0 is treated as false in conditions.

2. Step 2: Analyze the if condition behavior

   The condition if (strpos(...)) fails when strpos returns 0, so it wrongly goes to else.

3. Final Answer:

   strpos returns 0 for 'apple' position, which is treated as false -> Option B

4. Quick Check:

   Check strpos !== false to avoid 0 as false [OK]

Hint: Check strpos !== false, not just if() [OK]

Common Mistakes:

• Using if(strpos()) without strict false check
• Assuming strpos returns boolean true/false only
• Ignoring 0 as valid position

5. Given the string $sentence = 'The quick brown fox jumps', which code snippet correctly extracts the substring starting from the word 'brown' using strstr?

hard

A. echo strstr($sentence, 'brown');

B. echo strstr($sentence, 'brown', true);

C. echo strpos($sentence, 'brown');

D. echo substr($sentence, strpos($sentence, 'brown'));

Solution

1. Step 1: Understand strstr behavior

   strstr returns the part of the string starting from the first occurrence of the substring, including it.

2. Step 2: Analyze options

   echo strstr($sentence, 'brown'); correctly uses strstr to get substring from 'brown'. echo strstr($sentence, 'brown', true); returns part before 'brown'. echo strpos($sentence, 'brown'); returns position, not substring. echo substr($sentence, strpos($sentence, 'brown')); uses substr with strpos, which also works but is not strstr.

3. Final Answer:

   echo strstr($sentence, 'brown'); -> Option A

4. Quick Check:

   strstr returns substring from needle [OK]

Hint: strstr returns substring from needle, true returns before [OK]

Common Mistakes:

• Using third parameter true to get substring after needle
• Confusing strpos with strstr
• Expecting strpos to return substring