Implode and join in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code using implode?

Look at the PHP code below. What will it print?

PHP

<?php
$array = ['apple', 'banana', 'cherry'];
echo implode('-', $array);
?>

A apple-banana-cherry

B apple,banana,cherry

C apple banana cherry

D applebanana cherry

💡 Hint

implode joins array elements with the string you give as the first argument.

The implode function joins array elements into a string using the first argument as separator. Here, '-' is used, so the output is 'apple-banana-cherry'.

❓ Predict Output

intermediate

What does this PHP code output when using join()?

Check the PHP code below. What will it print?

PHP

<?php
$fruits = ['pear', 'grape', 'melon'];
echo join(', ', $fruits);
?>

A pear grape melon

B pear, grape, melon

C pear,grape,melon

D pear-grape-melon

💡 Hint

join() is an alias of implode(), so it works the same way.

join() works like implode(). The separator ', ' is used, so the output is 'pear, grape, melon'.

❓ Predict Output

advanced

What is the output of implode with a numeric array and no separator?

What will this PHP code print?

PHP

<?php
$numbers = [1, 2, 3, 4];
echo implode('', $numbers);
?>

A 1234

B 1 2 3 4

C 1,2,3,4

D Array

💡 Hint

If the separator is an empty string, elements join without spaces or commas.

implode with an empty string joins elements directly, so output is '1234'.

❓ Predict Output

advanced

What error or output does this code produce?

Consider this PHP code. What happens when you run it?

PHP

<?php
$items = ['a', 'b', 'c'];
echo implode($items);
?>

A abc

B a b c

C a,b,c

D Warning: implode(): Invalid arguments passed

💡 Hint

implode expects the separator first, then the array.

Calling implode with only one argument as an array causes a warning because the separator is missing.

🧠 Conceptual

expert

How many items are in the resulting string after implode?

Given this PHP code, how many characters will the output string have?

PHP

<?php
$words = ['hi', 'there', 'friend'];
$output = implode(' ', $words);
echo strlen($output);
?>

A 13

B 15

C 14

D 16

💡 Hint

Count letters plus spaces between words.

The words are 'hi' (2), 'there' (5), 'friend' (6). Spaces between words are 2. Total length = 2+1+5+1+6 = 14.

Practice

1. What does the PHP function implode do when used with an array?

easy

A. It splits a string into an array based on a separator.

B. It joins array elements into a single string with a separator.

C. It sorts the array elements alphabetically.

D. It removes duplicate elements from the array.

Solution

1. Step 1: Understand the purpose of implode

   The implode function takes an array and joins its elements into one string.

2. Step 2: Recognize the role of the separator

   You can specify a separator string to put between elements in the resulting string.

3. Final Answer:

   It joins array elements into a single string with a separator. -> Option B

4. Quick Check:

   implode joins array elements [OK]

Hint: Remember: implode combines array items into one string [OK]

Common Mistakes:

• Confusing implode with explode (which splits strings)
• Thinking implode sorts or filters arrays
• Assuming implode returns an array

2. Which of the following is the correct syntax to join array elements with a comma using implode?

easy

A. implode(',', $array);

B. implode($array, ',');

C. implode($array);

D. implode(',', array);

Solution

1. Step 1: Recall the correct parameter order of implode

   The first parameter is the separator string, the second is the array.

2. Step 2: Check each option's syntax

   implode(',', $array); uses implode(',', $array); which is correct syntax.

3. Final Answer:

   implode(',', $array); -> Option A

4. Quick Check:

   Separator first, array second in implode [OK]

Hint: Separator goes first, array second in implode [OK]

Common Mistakes:

• Swapping the order of parameters
• Using variable name without $ sign
• Omitting the array parameter

3. What is the output of the following PHP code?

$arr = ['apple', 'banana', 'cherry'];
echo implode(' - ', $arr);

medium

A. apple -banana -cherry

B. apple, banana, cherry

C. apple - banana - cherry

D. applebanana cherry

Solution

1. Step 1: Understand implode with ' - ' separator

   The separator string ' - ' is placed between each array element.

2. Step 2: Join the array elements

   Joining 'apple', 'banana', 'cherry' with ' - ' results in 'apple - banana - cherry'.

3. Final Answer:

   apple - banana - cherry -> Option C

4. Quick Check:

   implode with ' - ' joins elements with that exact separator [OK]

Hint: Check the separator string carefully in output [OK]

Common Mistakes:

• Confusing separator with comma
• Missing spaces around separator
• Assuming implode adds extra spaces

4. Identify the error in this PHP code snippet:

$fruits = ['apple', 'banana', 'cherry'];
echo implode($fruits, ', ');

medium

A. implode cannot be used with arrays.

B. The array variable is missing the $ sign.

C. The separator must be an array, not a string.

D. The separator and array parameters are swapped in implode.

Solution

1. Step 1: Check parameter order in implode

   implode expects separator first, then array second.

2. Step 2: Analyze given code

   Code uses implode($fruits, ', '); which swaps parameters.

3. Final Answer:

   The separator and array parameters are swapped in implode. -> Option D

4. Quick Check:

   Parameter order matters in implode [OK]

Hint: Separator first, array second in implode [OK]

Common Mistakes:

• Swapping parameters order
• Assuming implode works like explode
• Using wrong variable names

5. You have an array $words = ['one', '', 'three', null, 'five'];. You want to join only the non-empty, non-null elements with a comma. Which code correctly does this?

hard

A. $filtered = array_filter($words); echo implode(', ', $filtered);

B. echo implode(', ', $words);

C. $filtered = array_map(trim, $words); echo implode(', ', $filtered);

D. $filtered = array_filter($words, fn($w) => $w !== ''); echo implode(', ', $filtered);

Solution

1. Step 1: Remove empty and null elements using array_filter

   array_filter without callback removes falsy values like empty string and null.

2. Step 2: Use implode to join filtered array with comma

   Joining filtered elements with ', ' produces the desired string.

3. Final Answer:

   $filtered = array_filter($words); echo implode(', ', $filtered); -> Option A

4. Quick Check:

   array_filter removes empty/null, then implode joins [OK]

Hint: Filter falsy values before implode to exclude empties [OK]

Common Mistakes:

• Using implode directly without filtering
• Using array_map(trim) which doesn't remove null
• Filtering only empty strings but not null