Array merge and combine in PHP - Step-by-Step Execution

Concept Flow - Array merge and combine

Start with two arrays

↓

Choose operation: merge or combine

↓

Merge: append values

↓

Result: combined values, keys reindexed

↓

Combine: keys from first, values from second

↓

Result: new array with keys and values paired

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Use or display result

↓

End

Start with two arrays, then either merge by appending values or combine by pairing keys and values, then use the result.

Execution Sample

PHP

<?php
$array1 = ["a", "b"];
$array2 = ["c", "d"];
$merged = array_merge($array1, $array2);
$combined = array_combine($array1, $array2);
print_r($merged);
print_r($combined);
?>

This code merges two arrays by appending and combines them by pairing keys from the first with values from the second.

Execution Table

Step	Operation	Input Arrays	Result	Notes
1	Start	["a", "b"] and ["c", "d"]	N/A	Initial arrays
2	array_merge	["a", "b"], ["c", "d"]	["a", "b", "c", "d"]	Values appended, keys reindexed
3	array_combine	["a", "b"], ["c", "d"]	["a" => "c", "b" => "d"]	Keys from first array, values from second
4	End	N/A	Merged and combined arrays ready	Execution complete

💡 All operations complete, arrays merged and combined successfully

Variable Tracker

Variable	Start	After Merge	After Combine	Final
$array1	["a", "b"]	["a", "b"]	["a", "b"]	["a", "b"]
$array2	["c", "d"]	["c", "d"]	["c", "d"]	["c", "d"]
$merged	N/A	["a", "b", "c", "d"]	["a", "b", "c", "d"]	["a", "b", "c", "d"]
$combined	N/A	N/A	["a" => "c", "b" => "d"]	["a" => "c", "b" => "d"]

Key Moments - 3 Insights

Why does array_merge reindex keys instead of keeping original keys?

array_merge appends values and assigns new numeric keys starting from 0, as shown in step 2 of the execution_table, so keys are reindexed.

What happens if the first array has duplicate values when using array_combine?

array_combine uses the first array as keys, so duplicate values in the first array result in duplicate keys, but the last value overwrites previous ones for that key; no error is thrown.

Can array_combine be used if arrays have different lengths?

No, array_combine requires both arrays to have the same number of elements, otherwise it will produce a warning or error, as implied by the need for pairing in step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $merged after step 2?

A ["a" => "c", "b" => "d"]

B ["a", "b", "c", "d"]

C ["a", "b"]

D ["c", "d"]

💡 Hint

Check the 'Result' column for step 2 in the execution_table.

At which step does the array_combine operation occur?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Operation' column in the execution_table for array_combine.

If $array1 had 3 elements but $array2 had 2, what would happen to array_combine?

A It would ignore extra elements in $array1

B It would combine only first 2 pairs

C It would produce an error or warning

D It would merge arrays instead

💡 Hint

Recall from key_moments that array_combine requires arrays of equal length.

Concept Snapshot

array_merge($a1, $a2): appends values, reindexes keys
array_combine($a1, $a2): pairs keys from $a1 with values from $a2
Both require arrays as input
array_combine needs arrays of equal length
Use merge to join values, combine to create key-value pairs

Full Transcript

This visual execution trace shows how PHP's array_merge and array_combine functions work. Starting with two arrays, array_merge appends the values of the second array to the first and reindexes keys starting from zero. Array_combine pairs keys from the first array with values from the second array, creating a new associative array. The execution table traces each step, showing inputs and results. Key moments clarify common confusions like why keys are reindexed in merge and the requirement for equal array lengths in combine. The visual quiz tests understanding of these steps and behaviors. The concept snapshot summarizes the syntax and key rules for these functions.

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