String search functions (strpos, strstr) in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the strpos function do in PHP?

It finds the position of the first occurrence of a substring in a string. If the substring is found, it returns the index (starting at 0). If not found, it returns false.

beginner

What is the difference between strpos and strstr?

strpos returns the position (number) of the substring in the string. strstr returns the part of the string starting from the first occurrence of the substring to the end.

beginner

What does strstr return if the substring is not found?

It returns false if the substring is not found in the string.

intermediate

How does strpos handle the case when the substring is at the very start of the string?

It returns 0 because the substring starts at the first character (index 0). Remember, 0 is a valid position, not false.

intermediate

How can you safely check if a substring exists using strpos?

Use the strict comparison operator !== false because strpos can return 0 (which is falsey). For example: if (strpos($text, 'word') !== false) { /* found */ }

What does strpos('hello world', 'world') return?

A 6

B 0

C false

D 11

The substring 'world' starts at index 6 in 'hello world'.

What will strstr('hello world', 'world') return?

A 'world' plus the rest of the string starting from 'world'

B 'hello world'

C 'world' plus the rest of the string

D 'world'

strstr returns the part of the string starting from the first occurrence of the substring to the end, so it returns 'world'.

What does strpos('apple', 'a') return?

A false

B 0

C 1

D 5

The substring 'a' is at the start of 'apple', so the position is 0.

If strpos returns 0, what does that mean?

A Substring found at the end

B Substring not found

C Error in function

D Substring found at the start

0 means the substring is found at the very start of the string.

Which is the correct way to check if 'cat' is in a string $text?

A if (strpos($text, 'cat') == true) { ... }

B if (strpos($text, 'cat')) { ... }

C if (strpos($text, 'cat') !== false) { ... }

D if (strpos($text, 'cat') == false) { ... }

Use strict comparison !== false to correctly detect substring presence, even if at position 0.

Explain how strpos works and how to correctly check if a substring exists in a string.

Think about what happens if the substring is at the start.

Describe the behavior of strstr and what it returns when the substring is found or not found.

It returns a string, not a number.

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