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.