Array access and modification in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code?

Consider the following PHP code snippet:

$arr = [1, 2, 3, 4];
$arr[2] = $arr[0] + $arr[1];
echo $arr[2];

What will be printed?

PHP

$arr = [1, 2, 3, 4];
$arr[2] = $arr[0] + $arr[1];
echo $arr[2];

A 5

B 3

C 6

D Undefined offset error

💡 Hint

Remember that array indices start at 0 and you can assign new values to existing keys.

The original value at index 2 is 3, but it is replaced by the sum of values at indices 0 and 1, which are 1 and 2. So, 1 + 2 = 3, but since the original value was 3, the new value is 3. However, the code actually sums 1 + 2 = 3, so the output is 3.

❓ Predict Output

intermediate

What is the output after modifying a nested array?

Look at this PHP code:

$arr = ["a" => [1, 2], "b" => [3, 4]];
$arr["a"][1] = 5;
echo $arr["a"][1];

What will it print?

PHP

$arr = ["a" => [1, 2], "b" => [3, 4]];
$arr["a"][1] = 5;
echo $arr["a"][1];

A 5

B 2

C 1

D Undefined index error

💡 Hint

Think about how to access and change values inside nested arrays.

The code changes the second element (index 1) of the array under key "a" from 2 to 5. So echo prints 5.

🔧 Debug

advanced

Identify the error in this array modification code

What error does this PHP code produce?

$arr = [10, 20, 30];
$arr[3] += 5;
echo $arr[3];

PHP

$arr = [10, 20, 30];
$arr[3] += 5;
echo $arr[3];

A Outputs 5

B Notice: Undefined offset: 3

C Fatal error: Cannot use undefined offset

D Outputs 0

💡 Hint

Think about what happens when you try to add to an array element that does not exist yet.

Accessing $arr[3] before it is set triggers a notice about undefined offset. Then PHP treats it as 0 and adds 5, so echo prints 5, but the notice is shown.

📝 Syntax

advanced

Which option causes a syntax error in array modification?

Which of the following PHP code snippets will cause a syntax error?

A

$arr = [1, 2, 3];
$arr[0] = $arr[0] + 1;

B

$arr = [1, 2, 3];
$arr[] = 4;

C

$arr = [1, 2, 3];
$arr[2] +=;

D

$arr = [1, 2, 3];
$arr[1] = 5;

💡 Hint

Look carefully at the assignment operators and their usage.

Option C has an incomplete assignment operator '+=' without a value, causing a syntax error.

🚀 Application

expert

How many elements are in the array after these modifications?

Given this PHP code:

$arr = [];
$arr[2] = 'a';
$arr[] = 'b';
$arr[5] = 'c';
$arr[] = 'd';

How many elements does $arr have after these lines run?

PHP

$arr = [];
$arr[2] = 'a';
$arr[] = 'b';
$arr[5] = 'c';
$arr[] = 'd';

A 6

B 3

C 5

D 4

💡 Hint

Remember how PHP assigns keys when using empty brackets and how explicit keys affect the array.

After the first assignment, $arr has key 2. Then $arr[] adds 'b' at the next highest integer key, which is 3. Then key 5 is set to 'c'. Then $arr[] adds 'd' at key 6. So keys are 2,3,5,6 → 4 elements total.