Substring extraction in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP substring extraction?

Consider the following PHP code. What will it output?

PHP

<?php
$text = "Hello, world!";
echo substr($text, 7, 5);
?>

A world

B world!

C Hello

D o, wo

💡 Hint

Remember that substr starts at the offset and extracts the given length.

substr($text, 7, 5) starts at index 7 (0-based) and extracts 5 characters: 'world'.

❓ Predict Output

intermediate

What substring does this code extract?

What will be the output of this PHP code snippet?

PHP

<?php
$str = "Programming";
echo substr($str, -6, 3);
?>

A ram

B amm

C ogr

D ing

💡 Hint

Negative start means counting from the end of the string.

substr($str, -6, 3) starts 6 characters from the end and extracts 3 characters: 'amm'.

❓ Predict Output

advanced

What is the output when length exceeds string end?

What will this PHP code output?

PHP

<?php
$s = "OpenAI";
echo substr($s, 3, 10);
?>

A nAI

B nAI"

C enAI

D SyntaxError

💡 Hint

If length goes beyond string end, substr returns until the end.

substr($s, 3, 10) starts at index 3 and extracts till the end: 'nAI'.

❓ Predict Output

advanced

What happens if start is beyond string length?

What will this PHP code output?

PHP

<?php
$str = "Data";
echo substr($str, 10, 2);
?>

A ta

B Da

C Error: Out of bounds

D "" (empty string)

💡 Hint

If start is beyond string length, substr returns empty string.

substr($str, 10, 2) starts beyond string length, so returns empty string.

❓ Predict Output

expert

What is the output of this nested substring extraction?

Analyze the code and find the output:

PHP

<?php
$text = "abcdefg";
$part = substr($text, 2, 4);
echo substr($part, 1, 2);
?>

A cd

B bc

C de

D ef

💡 Hint

First extract 4 chars from index 2, then extract 2 chars from index 1 of that result.

substr($text, 2, 4) is 'cdef'. Then substr('cdef', 1, 2) is 'de'.

Practice

1. What does the PHP function substr($text, 2, 3) do?

easy

A. Extracts the first 3 characters of $text

B. Extracts 2 characters from $text starting at index 3

C. Extracts 3 characters from $text starting at index 2

D. Extracts the last 3 characters of $text

Solution

1. Step 1: Understand substr parameters

   The first parameter is the string, the second is the start index (0-based), and the third is the length of substring.

2. Step 2: Apply to given function

   Starting at index 2 means the third character, and length 3 means extract 3 characters from there.

3. Final Answer:

   Extracts 3 characters from $text starting at index 2 -> Option C

4. Quick Check:

   substr(string, 2, 3) = 3 chars from index 2 [OK]

Hint: Remember start is zero-based, length is how many chars [OK]

Common Mistakes:

• Confusing start index with length
• Thinking start counts from 1 instead of 0
• Assuming length is mandatory

2. Which of the following is the correct syntax to extract a substring from a string $str starting at position 4 with length 5 in PHP?

easy

A. substr($str, 5);

B. substr($str, 4, 5);

C. substr($str, 4);

D. substr($str, 5, 4);

Solution

1. Step 1: Recall substr syntax

   substr(string, start, length) extracts substring starting at 'start' index with 'length' characters.

2. Step 2: Match parameters to question

   Start is 4, length is 5, so substr($str, 4, 5) is correct syntax.

3. Final Answer:

   substr($str, 4, 5); -> Option B

4. Quick Check:

   substr($str, 4, 5) = correct syntax [OK]

Hint: Syntax is substr(string, start, length) with start zero-based [OK]

Common Mistakes:

• Swapping start and length values
• Omitting length when needed
• Using 1-based start index

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

$text = "Hello World!";
echo substr($text, 6, 5);

medium

A. World

B. World!

C. Hello

D. orld!

Solution

1. Step 1: Identify substring start and length

   Start index is 6 (0-based), length is 5 characters.

2. Step 2: Extract substring from "Hello World!"

   Index 6 is 'W', next 5 chars are 'World'.

3. Final Answer:

   World -> Option A

4. Quick Check:

   substr("Hello World!", 6, 5) = "World" [OK]

Hint: Count characters from zero, then take length chars [OK]

Common Mistakes:

• Including the exclamation mark mistakenly
• Starting count from 1 instead of 0
• Confusing length with end index

4. The following PHP code is intended to extract the substring "cat" from the string "concatenate". What is wrong?

$word = "concatenate";
echo substr($word, 3, 4);

medium

A. The length parameter is too long; it should be 3

B. The start index should be 4 instead of 3

C. The substr function cannot extract "cat" from this string

D. There is no error; the code works correctly

Solution

1. Step 1: Locate substring "cat" in "concatenate"

   "cat" starts at index 3: c(3), a(4), t(5).

2. Step 2: Check length parameter

   Length 4 extracts "cate" (4 chars), but "cat" is only 3 chars, so length should be 3.

3. Final Answer:

   The length parameter is too long; it should be 3 -> Option A

4. Quick Check:

   substr("concatenate", 3, 3) = "cat" [OK]

Hint: Count exact substring length to avoid extra chars [OK]

Common Mistakes:

• Using wrong start index
• Assuming substr auto-trims extra chars
• Thinking substr can't extract middle parts

5. You have a string $data = "user:john;id:12345;role:admin";. How do you extract the user name "john" using substr and strpos functions in PHP?

hard

A. substr($data, 0, strpos($data, ':'));

B. substr($data, 5, 4);

C. substr($data, strpos($data, ';') + 1, 4);

D. substr($data, strpos($data, ':') + 1, strpos($data, ';') - strpos($data, ':') - 1);

Solution

1. Step 1: Find start position of username

   Use strpos($data, ':') to find colon after "user", add 1 to start after colon.

2. Step 2: Find length of username substring

   Find position of first semicolon with strpos($data, ';'), subtract start position and 1 to get length.

3. Step 3: Use substr with calculated start and length

   substr($data, start, length) extracts "john" correctly.

4. Final Answer:

   substr($data, strpos($data, ':') + 1, strpos($data, ';') - strpos($data, ':') - 1); -> Option D

5. Quick Check:

   Use strpos to find delimiters, substr extracts between them [OK]

Hint: Use strpos to find delimiters, substr extracts between them [OK]

Common Mistakes:

• Hardcoding start and length without dynamic positions
• Using wrong delimiter positions
• Extracting wrong substring part