Array merge and combine in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the array_merge() function do in PHP?

It joins two or more arrays into one array by appending the elements of later arrays to the first array. If keys are numeric, they are re-indexed; if keys are strings, later values overwrite earlier ones.

beginner

How does array_combine() work in PHP?

It creates a new array by using one array for keys and another array for values. Both arrays must have the same number of elements.

intermediate

What happens if the arrays passed to array_combine() have different lengths?

PHP will return FALSE because the function requires both arrays to have the same number of elements to pair keys and values correctly.

beginner

Given two arrays $a = [1, 2] and $b = [3, 4], what is the result of array_merge($a, $b)?

The result is [1, 2, 3, 4]. The elements of $b are appended to $a and re-indexed numerically.

beginner

How can you combine keys and values arrays into an associative array in PHP?

Use array_combine($keys, $values) where $keys is an array of keys and $values is an array of values. The result is an associative array pairing each key with its value.

What does array_merge() do when merging arrays with string keys?

A Later values overwrite earlier ones for the same keys

B It appends values and keeps all keys duplicated

C It ignores string keys and only merges numeric keys

D It throws an error

When merging arrays with string keys, array_merge() overwrites earlier values with later ones for the same keys.

What must be true for array_combine() to work correctly?

A Both arrays must have unique values

B Both arrays must have numeric keys

C Both arrays must be empty

D Both arrays must have the same number of elements

array_combine() requires both arrays to have the same number of elements to pair keys and values.

What will array_merge(["a" => 1], ["a" => 2]) return?

A ["a" => 2]

B ["a" => 1, "a" => 2]

C [1, 2]

D An error

The second array's value overwrites the first for the same string key "a".

What does array_combine(["x", "y"], [10, 20]) produce?

A [10, 20]

B ["x" => 10, "y" => 20]

C ["x", "y", 10, 20]

D FALSE

It creates an associative array with keys "x" and "y" paired to values 10 and 20.

What happens if you pass arrays of different sizes to array_combine()?

A It fills missing values with NULL

B It merges arrays ignoring extra elements

C It returns FALSE

D It throws a warning but returns partial array

PHP returns FALSE if the arrays have different lengths because it cannot pair keys and values properly.

Explain how array_merge() works when combining two arrays with numeric and string keys.

Think about how PHP handles keys differently based on their type.

Describe the requirements and result of using array_combine() in PHP.

Consider what happens if the arrays differ in size.

Practice

1. What does the PHP function array_merge do?

easy

A. Creates an associative array from two arrays

B. Joins two or more arrays into one combined array

C. Sorts an array in ascending order

D. Removes duplicate values from an array

Solution

1. Step 1: Understand the purpose of array_merge

   The function array_merge takes multiple arrays and joins their elements into a single array.

2. Step 2: Compare with other functions

   array_combine creates key-value pairs, sorting and removing duplicates are different functions.

3. Final Answer:

   Joins two or more arrays into one combined array -> Option B

4. Quick Check:

   array_merge joins arrays = B [OK]

Hint: array_merge joins arrays end-to-end [OK]

Common Mistakes:

• Confusing array_merge with array_combine
• Thinking array_merge sorts arrays
• Assuming array_merge removes duplicates

2. Which of the following is the correct syntax to combine two arrays into an associative array using array_combine?

easy

A. array_merge($keys, $values);

B. array_combine($values, $keys);

C. array_combine($keys, $values);

D. array_combine($keys);

Solution

1. Step 1: Recall array_combine parameter order

   The first parameter is the array of keys, the second is the array of values.

2. Step 2: Check each option

   array_combine($keys, $values); correctly uses array_combine($keys, $values);. array_combine($values, $keys); reverses parameters, C uses wrong function, D misses second parameter.

3. Final Answer:

   array_combine($keys, $values); -> Option C

4. Quick Check:

   Keys first, values second in array_combine = A [OK]

Hint: Keys go first, values second in array_combine [OK]

Common Mistakes:

• Swapping keys and values parameters
• Using array_merge instead of array_combine
• Missing one of the required parameters

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

$a = [1, 2];
$b = [3, 4];
$result = array_merge($a, $b);
print_r($result);

medium

A. [1, 2, 3, 4]

B. [3, 4, 1, 2]

C. [1 => 2, 3 => 4]

D. Array keys and values combined

Solution

1. Step 1: Understand array_merge behavior

   array_merge joins arrays by appending elements of second array to first, preserving order.

2. Step 2: Apply to given arrays

   $a = [1, 2], $b = [3, 4], merged result is [1, 2, 3, 4].

3. Final Answer:

   [1, 2, 3, 4] -> Option A

4. Quick Check:

   array_merge appends arrays = [1, 2, 3, 4] [OK]

Hint: array_merge appends arrays in order given [OK]

Common Mistakes:

• Thinking array_merge sorts or combines keys
• Confusing output with associative array
• Expecting keys to be preserved as in original arrays

4. Identify the error in this PHP code snippet:

$keys = ['a', 'b'];
$values = [1];
$result = array_combine($keys, $values);
print_r($result);

medium

A. array_combine merges arrays instead of combining keys and values

B. array_combine cannot use string keys

C. array_combine needs only one parameter

D. array_combine requires both arrays to have the same number of elements

Solution

1. Step 1: Check array_combine requirements

   array_combine requires both arrays to have equal length to pair keys and values correctly.

2. Step 2: Analyze given arrays

   $keys has 2 elements, $values has 1 element, causing an error.

3. Final Answer:

   array_combine requires both arrays to have the same number of elements -> Option D

4. Quick Check:

   Equal length arrays needed for array_combine = D [OK]

Hint: Keys and values arrays must be same length [OK]

Common Mistakes:

• Ignoring array length mismatch
• Thinking array_combine works with one array
• Confusing array_combine with array_merge

5. Given two arrays:
$keys = ['name', 'age', 'city'];
$values = ['Alice', 30, 'Paris'];
Which PHP code correctly creates an associative array and then merges it with ['country' => 'France']?

hard

A. $assoc = array_combine($keys, $values); $final = array_merge($assoc, ['country' => 'France']);

B. $assoc = array_merge($keys, $values); $final = array_combine($assoc, ['country' => 'France']);

C. $final = array_combine($keys, $values, ['country' => 'France']);

D. $final = array_merge(array_combine($values, $keys), ['country' => 'France']);

Solution

1. Step 1: Create associative array from keys and values

   Use array_combine($keys, $values) to pair keys with values correctly.

2. Step 2: Merge associative array with another array

   Use array_merge to add the 'country' key-value pair to the associative array.

3. Step 3: Check other options for errors

   $assoc = array_merge($keys, $values); $final = array_combine($assoc, ['country' => 'France']); swaps functions incorrectly, C uses wrong parameter count, D swaps keys and values.

4. Final Answer:

   $assoc = array_combine($keys, $values); $final = array_merge($assoc, ['country' => 'France']); -> Option A

5. Quick Check:

   Combine keys-values, then merge extra array = A [OK]

Hint: Combine keys-values first, then merge extra pairs [OK]

Common Mistakes:

• Swapping keys and values in array_combine
• Trying to combine more than two arrays at once
• Using array_merge before array_combine