Can array_flip() support null, false, and empty string values?

M66 2025-06-03

In PHP, the array_flip() function is used to swap keys and values in an array. Its basic function is to take the keys of an array as values and the value of an array as keys. Suppose there is an array like this:

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

After calling array_flip($array) , the result will be:

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

During the use of array_flip() , if the value of the array contains some special type of data, such as null , false , or empty string, it may affect the flipped result. Below we will discuss the behavior of these three special values in array_flip() in detail.

1. null value

If a value in the array is null , array_flip() will take null as the key. Specifically, null in PHP will be processed as an empty string. In other words, multiple null values will result in the same key, and eventually only one key will be retained. Therefore, if there are duplicate null values in the array, the flipped result will only have one key.

 $array = [
    'a' => null,
    'b' => null
];

$flippedArray = array_flip($array);
print_r($flippedArray);

The output result is:

 Array
(
    [0] => b
)

As you can see, although there are two null values in the original array, after flip, null is treated as 0 and b overrides a .

2. false value

For false value, array_flip() converts it to integer 0 . Similar to null , false is treated as a key with a specific value, so if there are multiple false values in the array, there will be a key conflict after flipping, resulting in only the last key-value pair being retained.

 $array = [
    'a' => false,
    'b' => false
];

$flippedArray = array_flip($array);
print_r($flippedArray);

The output result is:

 Array
(
    [0] => b
)

Here, false is converted to 0 , b overwrites a , so the flipped array has only one key 0 .

3. Empty string ( "" ) value

When an empty string ( "" ) is used as an array value, array_flip() treats it as a key. Since empty strings can be used as keys like other values, if there are multiple empty string values in the array, they will also conflict, and in the end only the key of the last empty string is preserved.

 $array = [
    'a' => '',
    'b' => ''
];

$flippedArray = array_flip($array);
print_r($flippedArray);

The output result is:

 Array
(
    [] => b
)

In the flipped array, an empty string is treated as a key ( [] means an empty string), and b overwrites a .

Summarize

The behavior of array_flip() when encountering a special value is summarized as follows:

The null value will be treated as 0 , and multiple null values will cause key conflicts, leaving only the last key.
The false value will be converted to an integer 0 , and multiple false values will also cause key conflicts, leaving the last key.
When an empty string ( "" ) is used as a value, it will be regarded as a key. Multiple empty strings will have key conflicts, and only the key of the last empty string will be retained.

Therefore, when using array_flip() , if the array contains these special values, special attention should be paid to the issue of key conflicts. To avoid unnecessary conflicts, it is recommended to check the type of array value before using array_flip() to ensure there are no undesirable special values.

array_flip