Array access and modification in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you access the first element of an array in PHP?

Use the index 0 inside square brackets after the array variable. For example: $array[0] accesses the first element.

beginner

How do you change the value of an element in a PHP array?

Assign a new value to the element using its index or key. For example: $array[2] = 'new value'; changes the third element.

intermediate

What happens if you assign a value to an array index that does not exist yet?

PHP will add a new element at that index with the assigned value. This can create new elements dynamically.

beginner

How do you add a new element to the end of an array without specifying an index?

Use empty square brackets to append: $array[] = 'new element'; adds the element at the end.

beginner

How do you access an element in an associative array by key?

Use the key inside square brackets as a string. For example: $array['key'] accesses the element with that key.

What does $array[1] return in PHP?

A The first element of the array

B The second element of the array

C The last element of the array

D An error

Array indexes start at 0, so index 1 is the second element.

How do you add a new element to the end of an array without specifying the index?

A Both A and C

B $array[] = 'value';

C array_push($array, 'value');

D $array[0] = 'value';

Both $array[] = 'value'; and array_push($array, 'value'); add elements to the end.

What happens if you assign a value to $array[5] when the array has only 3 elements?

A PHP adds a new element at index 5

B PHP throws an error

C PHP replaces the third element

D PHP ignores the assignment

PHP creates a new element at index 5, leaving indexes 3 and 4 undefined.

How do you access the value associated with the key 'name' in an associative array?

A $array.name

B $array[name]

C $array->name

D $array['name']

Keys in associative arrays must be in quotes inside square brackets.

Which of the following is the correct way to change the third element of an array to 10?

A $array(2) = 10;

B $array[3] = 10;

C $array[2] = 10;

D $array->2 = 10;

Arrays start at index 0, so the third element is at index 2.

Explain how to access and modify elements in a PHP array.

Think about how you use square brackets with the array variable.

Describe what happens when you assign a value to an array index that does not exist yet in PHP.

PHP arrays are flexible and can grow as you assign new indexes.

Practice

1. What is the correct way to access the first element of an array $arr in PHP?

easy

A. $arr{0}

B. $arr[1]

C. $arr[0]

D. $arr->0

Solution

1. Step 1: Understand PHP array indexing

   PHP arrays start at index 0, so the first element is at index 0.

2. Step 2: Identify correct syntax for array access

   Use square brackets with the index number: $arr[0].

3. Final Answer:

   $arr[0] -> Option C

4. Quick Check:

   First element index = 0 [OK]

Hint: First element is always at index 0 in PHP arrays [OK]

Common Mistakes:

• Using index 1 for first element
• Using curly braces for array access
• Using object property syntax

2. Which of the following is the correct way to change the value of the third element in array $arr to 10?

easy

A. $arr(3) = 10;

B. $arr[2] = 10;

C. $arr->2 = 10;

D. $arr{3} = 10;

Solution

1. Step 1: Identify the index of the third element

   Arrays start at 0, so the third element is at index 2.

2. Step 2: Use correct syntax to assign value

   Use square brackets with index 2: $arr[2] = 10;.

3. Final Answer:

   $arr[2] = 10; -> Option B

4. Quick Check:

   Index 2 for third element assignment [OK]

Hint: Use square brackets and zero-based index to modify array [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using object property syntax
• Using wrong index number

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

$arr = [5, 10, 15];
$arr[1] = $arr[1] + 5;
echo $arr[1];

medium

A. 15

B. 10

C. 20

D. 5

Solution

1. Step 1: Understand initial array values

   $arr = [5, 10, 15]; so $arr[1] is 10.

2. Step 2: Calculate new value of $arr[1]

   $arr[1] = $arr[1] + 5 means 10 + 5 = 15.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   10 + 5 = 15 [OK]

Hint: Add to existing value using $arr[index] + number [OK]

Common Mistakes:

• Printing old value without update
• Adding index instead of value
• Confusing array indexes

4. Identify the error in the following PHP code snippet:

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

medium

A. Accessing an undefined index causes an error

B. Cannot echo array elements directly

C. Syntax error in array declaration

D. Assigning to index 3 adds a new element without error

Solution

1. Step 1: Check array indexes and assignment

   Array has indexes 0,1,2 initially. Assigning $arr[3] = 4 adds a new element at index 3.

2. Step 2: Confirm if this causes error

   PHP allows adding new elements by assigning to new indexes without error.

3. Final Answer:

   Assigning to index 3 adds a new element without error -> Option D

4. Quick Check:

   PHP arrays auto-expand on new index assignment [OK]

Hint: Assigning new index adds element; no error in PHP [OK]

Common Mistakes:

• Thinking accessing undefined index causes error
• Confusing syntax error with runtime behavior
• Believing echo can't print array elements

5. Given the array $arr = [2, 4, 6, 8];, which code snippet correctly doubles each element's value by modifying the array in place?

hard

A. for ($i = 0; $i < count($arr); $i++) { $arr[$i] *= 2; }

B. $arr = array_fill(0, count($arr), 2 * $arr[0]);

C. array_map(fn($v) => $v * 2, $arr);

D. foreach ($arr as $value) { $value = $value * 2; }

Solution

1. Step 1: Understand how to modify array elements in place

   Using a for loop with index allows direct modification of each element.

2. Step 2: Analyze each option

   A: foreach copies values, does not modify original array.
   B: array_fill replaces all elements with same value, incorrect.
   C: array_map returns new array, original unchanged.
   D: for loop modifies $arr[$i] directly.

3. Final Answer:

   for ($i = 0; $i < count($arr); $i++) { $arr[$i] *= 2; } -> Option A

4. Quick Check:

   Use indexed loop to modify array elements [OK]

Hint: Use for loop with indexes to change array elements directly [OK]

Common Mistakes:

• Modifying foreach loop variable instead of array
• Expecting array_map to change original array
• Using array_fill incorrectly