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associative array
array manipulation
two-dimensional array
flip array

Flip two-dimensional associative array in PHP

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Introduction

Flipping a two-dimensional associative array in PHP means transposing it — converting rows to columns and columns to rows. If the original array maps outer keys to inner key-value pairs, the flipped version maps inner keys to outer key-value pairs. PHP does not have a built-in array_transpose() function, so you build it by iterating through the nested array and reorganizing the keys. The array_flip() function only works on one-dimensional arrays and is not suitable for this operation.

What Flipping Means

Given a 2D array structured as a table:

php
1$data = [
2    'Alice' => ['math' => 90, 'science' => 85, 'english' => 92],
3    'Bob'   => ['math' => 78, 'science' => 88, 'english' => 76],
4    'Carol' => ['math' => 95, 'science' => 91, 'english' => 89],
5];

Flipping (transposing) converts person → subject to subject → person:

php
1$flipped = [
2    'math'    => ['Alice' => 90, 'Bob' => 78, 'Carol' => 95],
3    'science' => ['Alice' => 85, 'Bob' => 88, 'Carol' => 91],
4    'english' => ['Alice' => 92, 'Bob' => 76, 'Carol' => 89],
5];

Method 1: Nested foreach Loop

php
1$data = [
2    'Alice' => ['math' => 90, 'science' => 85, 'english' => 92],
3    'Bob'   => ['math' => 78, 'science' => 88, 'english' => 76],
4    'Carol' => ['math' => 95, 'science' => 91, 'english' => 89],
5];
6
7$flipped = [];
8foreach ($data as $outerKey => $innerArray) {
9    foreach ($innerArray as $innerKey => $value) {
10        $flipped[$innerKey][$outerKey] = $value;
11    }
12}
13
14print_r($flipped);
15// Array (
16//     [math] => Array ( [Alice] => 90, [Bob] => 78, [Carol] => 95 )
17//     [science] => Array ( [Alice] => 85, [Bob] => 88, [Carol] => 91 )
18//     [english] => Array ( [Alice] => 92, [Bob] => 76, [Carol] => 89 )
19// )

This is the most straightforward and readable approach.

Method 2: Reusable Function

php
1function array_transpose(array $array): array
2{
3    $result = [];
4    foreach ($array as $outerKey => $innerArray) {
5        foreach ($innerArray as $innerKey => $value) {
6            $result[$innerKey][$outerKey] = $value;
7        }
8    }
9    return $result;
10}
11
12$scores = [
13    'Q1' => ['revenue' => 1000, 'costs' => 700, 'profit' => 300],
14    'Q2' => ['revenue' => 1200, 'costs' => 750, 'profit' => 450],
15    'Q3' => ['revenue' => 1100, 'costs' => 680, 'profit' => 420],
16];
17
18$byMetric = array_transpose($scores);
19// $byMetric['revenue'] => ['Q1' => 1000, 'Q2' => 1200, 'Q3' => 1100]

Method 3: array_column (Numeric Outer Keys)

For arrays with numeric outer keys and consistent inner keys, array_column() extracts a single column:

php
1$users = [
2    ['name' => 'Alice', 'age' => 30, 'city' => 'NYC'],
3    ['name' => 'Bob',   'age' => 25, 'city' => 'LA'],
4    ['name' => 'Carol', 'age' => 35, 'city' => 'Chicago'],
5];
6
7// Extract a single column
8$names = array_column($users, 'name');
9// ['Alice', 'Bob', 'Carol']
10
11// Extract with a custom key
12$ageByName = array_column($users, 'age', 'name');
13// ['Alice' => 30, 'Bob' => 25, 'Carol' => 35]
14
15// Full transpose using array_column for each key
16$keys = array_keys($users[0]);
17$transposed = [];
18foreach ($keys as $key) {
19    $transposed[$key] = array_column($users, $key);
20}
21// $transposed['name'] => ['Alice', 'Bob', 'Carol']
22// $transposed['age'] => [30, 25, 35]

Method 4: array_map with Spread Operator

For numerically-indexed 2D arrays:

php
1$matrix = [
2    [1, 2, 3],
3    [4, 5, 6],
4    [7, 8, 9],
5];
6
7// Transpose using array_map with null callback
8$transposed = array_map(null, ...$matrix);
9
10print_r($transposed);
11// [[1, 4, 7], [2, 5, 8], [3, 6, 9]]

This uses the special behavior of array_map(null, ...) which zips arrays together. It only works with numeric keys.

Flipping 1D Arrays with array_flip

For simple one-dimensional arrays, array_flip() swaps keys and values:

php
1$colors = ['r' => 'red', 'g' => 'green', 'b' => 'blue'];
2
3$flipped = array_flip($colors);
4// ['red' => 'r', 'green' => 'g', 'blue' => 'b']

array_flip() does not work on 2D arrays — it only handles scalar values.

Handling Uneven Inner Arrays

When inner arrays have different keys:

php
1$data = [
2    'Alice' => ['math' => 90, 'science' => 85],
3    'Bob'   => ['math' => 78, 'english' => 76],  // Missing 'science', has 'english'
4];
5
6$flipped = array_transpose($data);
7// [
8//     'math'    => ['Alice' => 90, 'Bob' => 78],
9//     'science' => ['Alice' => 85],           // Bob missing
10//     'english' => ['Bob' => 76],             // Alice missing
11// ]
12
13// Fill missing values with a default
14function array_transpose_filled(array $array, $default = null): array
15{
16    $allInnerKeys = [];
17    foreach ($array as $inner) {
18        $allInnerKeys = array_merge($allInnerKeys, array_keys($inner));
19    }
20    $allInnerKeys = array_unique($allInnerKeys);
21
22    $result = [];
23    foreach ($allInnerKeys as $innerKey) {
24        foreach ($array as $outerKey => $inner) {
25            $result[$innerKey][$outerKey] = $inner[$innerKey] ?? $default;
26        }
27    }
28    return $result;
29}

Common Pitfalls

  • Using array_flip() on a 2D array: array_flip() only works with scalar values (strings and integers). Passing a nested array triggers a warning and produces incorrect results. Use a custom transpose function instead.
  • Assuming all inner arrays have the same keys: If inner arrays have different keys, some entries in the transposed result will be missing. Check for key consistency or provide a default value for missing entries.
  • Using array_map(null, ...$array) with associative keys: The array_map(null, ...) trick only works with numerically-indexed arrays. Associative keys are discarded during the spread operation.
  • Duplicate values in array_flip() for 1D arrays: When flipping a 1D array, duplicate values cause data loss because the last occurrence overwrites earlier ones. Use array_count_values() or group manually if duplicates exist.
  • Mutating the original array: None of these methods modify the original array — they all return new arrays. If you assign the result back to the same variable, the original data is lost.

Summary

  • Transpose a 2D associative array by iterating with nested foreach and swapping outer/inner keys
  • Use array_column() to extract specific columns from arrays of associative arrays
  • Use array_map(null, ...$array) to transpose numerically-indexed 2D arrays
  • array_flip() only works on 1D arrays with scalar values — it swaps keys and values
  • Handle uneven inner arrays by collecting all keys first and filling missing values with a default
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