array_flip() What happens when a duplicate value is encountered?

M66 2025-05-14

In PHP, array_flip() is a very practical function, and its function is to put arrays in. In other words, the original value will become a new key, and the original key will become a new value.

But the question is: What if there are duplicate values in the original array? How will array_flip() be handled? Will the previous value be overwritten? This article will analyze this issue in detail.

Basic usage of array_flip()

Let’s take a look at a basic example:

 $input = [
    'a' => 1,
    'b' => 2,
    'c' => 3
];

$flipped = array_flip($input);
print_r($flipped);

Output result:

 Array
(
    [1] => a
    [2] => b
    [3] => c
)

This is OK, because the values are unique and can be safely used as keys.

Repeat value situation

Let's look at another example, this time with a duplicate value:

 $input = [
    'first'  => 'apple',
    'second' => 'banana',
    'third'  => 'apple'
];

$flipped = array_flip($input);
print_r($flipped);

Output result:

 Array
(
    [banana] => second
    [apple] => third
)

As you can see, the key 'first' => 'apple' is overwritten by 'third' => 'apple' after flip. That is to say, array_flip() will use the last occurrence value as the final key , and the previous same-value items will be ignored (overridden).

This should be paid special attention when processing big data to avoid accidental loss of data.

Practical application scenarios

Suppose you are developing a tag system, with each tag ID corresponding to a tag name:

 $tags = [
    1 => 'php',
    2 => 'javascript',
    3 => 'php'
];

$flipped = array_flip($tags);

You may want to back-check the ID by tag name, but at this time 'php' will only retain one ID ( 3 ) and ID 1 will be overwritten. To handle this situation safely, you may need to use other methods, such as manual traversal:

 $reverse = [];
foreach ($tags as $id => $name) {
    $reverse[$name][] = $id;
}

This allows all IDs to be retained and becomes a many-to-one relationship.

Tips: Pay attention to the legality of keys

Since the keys of an array in PHP must be integers or strings , if the value is an array, object, or null , etc. that cannot be used as a key, array_flip() will throw a warning:

 $input = [
    'a' => null,
    'b' => ['nested']
];

$flipped = array_flip($input); // Will report an error

Summarize

array_flip() will take the value as the key and the key as the value.
When a duplicate value is encountered, the front will be overwritten by the back.
To avoid data loss, consider using more flexible processing methods such as grouping.
Note that the value must be of the type that can be used as a key.

array_flip