Substring extraction in PHP

Introduction

Substring extraction helps you get a smaller part of a bigger text. It is like cutting out a piece of a sentence you want to use.

You want to get the first name from a full name string.

You need to extract the area code from a phone number.

You want to get a file extension from a filename.

You want to show only a preview of a long text.

Syntax

PHP

<?php
// Extract substring from a string
$substring = substr($string, $start, $length = null);
?>

$string is the original text.

$start is where to start cutting (0 means start of string).

$length is how many characters to take (optional).

Examples

Extracts 5 characters starting at position 6 (counting from 0).

PHP

<?php
$text = "Hello World!";
$part = substr($text, 6, 5);
echo $part; // Outputs: World
?>

Extracts from position 6 to the end of the string.

PHP

<?php
$text = "Hello World!";
$part = substr($text, 6);
echo $part; // Outputs: World!
?>

If length is longer than string, it returns till end without error.

PHP

<?php
$text = "Hello";
$part = substr($text, 0, 10);
echo $part; // Outputs: Hello
?>

Negative start counts from the end of the string.

PHP

<?php
$text = "Hello";
$part = substr($text, -3, 2);
echo $part; // Outputs: ll
?>

Sample Program

This program shows different ways to get parts of a string using substr. It prints the original text, then extracts a small word, a part from a position to the end, the last 11 characters, and tries to extract more characters than the string length.

PHP

<?php
// Original string
$full_text = "Learning PHP substring extraction.";

// Show original string
echo "Original text: $full_text\n";

// Extract 'PHP' from the string
$extracted_part = substr($full_text, 9, 3);
echo "Extracted part: $extracted_part\n";

// Extract from position 9 to end
$extracted_to_end = substr($full_text, 9);
echo "From position 9 to end: $extracted_to_end\n";

// Extract last 11 characters
$last_eleven = substr($full_text, -11);
echo "Last 11 characters: $last_eleven\n";

// Extract with length longer than string
$long_extract = substr($full_text, 0, 100);
echo "Long extract: $long_extract\n";
?>

Important Notes

Time complexity is O(n) where n is the length of the substring extracted.

Space complexity is O(n) for the new substring created.

Common mistake: forgetting that string positions start at 0, so counting starts from zero.

Use substring extraction when you need a specific part of text. For splitting by a character, consider explode() instead.

Summary

Substring extraction gets a smaller piece from a bigger string.

Use substr(string, start, length) to cut out the part you want.

Start counts from zero, and length is optional to go till the end.

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