PHP array_flip Function

last modified March 13, 2025

The PHP array_flip function exchanges all keys with their associated values in an array. It's useful for reversing key-value pairs.

Basic Definition

The array_flip function swaps array keys and values. The original values become keys, and keys become values in the returned array.

Syntax: array_flip(array $array): array. Values must be valid keys (string or integer). Duplicate values will be overwritten.

Basic array_flip Example

This demonstrates a simple key-value swap in an associative array.

basic_array_flip.php

<?php

$colors = [
    'red' => '#FF0000',
    'green' => '#00FF00',
    'blue' => '#0000FF'
];

$flipped = array_flip($colors);

print_r($flipped);

Output: The hex color codes become keys, and color names become values. Original keys must be strings or integers, and values must be valid keys.

Handling Duplicate Values

When duplicate values exist, the last key wins in the flipped array.

duplicate_values.php

<?php

$fruitCounts = [
    'apples' => 5,
    'oranges' => 3,
    'bananas' => 5
];

$flipped = array_flip($fruitCounts);

print_r($flipped);

Output shows only one '5' key (from bananas) since apples also had value 5. The last occurrence of a value determines which key is kept in the result.

Numeric Array Flipping

Flipping a numeric array effectively creates a value-to-index mapping.

numeric_array.php

<?php

$numbers = [10, 20, 30];
$flipped = array_flip($numbers);

print_r($flipped);

Output shows the original values as keys with their original indices as values. This creates an inverted index useful for reverse lookups.

Type Requirements

Values must be valid keys (string or integer) or array_flip will warn.

type_requirements.php

<?php

$mixed = [
    'a' => 1,
    'b' => 'two',
    'c' => ['invalid']
];

$flipped = @array_flip($mixed); // Suppress warning

print_r($flipped);

Output shows only valid key-value pairs. The array element is skipped with a warning since arrays can't be keys. Use error suppression carefully.

Practical Use Case

Flipping can create efficient value-to-key lookups for validation.

practical_use.php

<?php

$allowedRoles = ['admin', 'editor', 'viewer'];
$roleLookup = array_flip($allowedRoles);

$userRole = 'editor';

if (isset($roleLookup[$userRole])) {
    echo "Access granted";
} else {
    echo "Access denied";
}

This technique provides O(1) lookup time compared to in_array's O(n). Flipped arrays are memory-efficient for checking value existence.

Best Practices

Valid Keys: Ensure all values can be valid array keys.
Unique Values: Be aware of duplicate value behavior.
Memory: Flipping large arrays consumes memory.
Lookups: Use flipped arrays for fast value checks.

Source

PHP array_flip Documentation

This tutorial covered PHP's array_flip function with practical examples showing key-value swapping and common use cases.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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