How to check if an array is associative or sequential in PHP?
An array is sequential if its keys match a continuous integer sequence starting from 0, determined by comparing array_keys($array) with array_keys(array_keys($array)); otherwise, it is associative.
To check if an array is associative or sequential in PHP, you need to examine the array's keys. A sequential array has integer keys in order starting from 0, while an associative array contains non-integer keys or integer keys that are not in sequence starting from 0.
Check for Sequential Array
A reliable way to determine if an array is sequential is to compare its keys with a generated range of integers from 0 to the count of elements minus one.
- Use array_keys() to get all the keys of the array.
- Check if all keys are integers and match the expected sequence.
Here’s a function to check if an array is sequential:
function isSequential($array) {
$keys = array_keys($array);
return $keys === array_keys($keys);
}
This works because array_keys($keys) generates a sequential integer array (0, 1, 2, ...), and if the original keys match this, the array is sequential.
Check for Associative Array
An associative array fails the sequential test. So you can define an associative array as one that is not sequential.
- If the keys are not a continuous sequence starting from 0, it's considered associative.
- Arrays with string keys or gaps in integer keys will return false in the sequential check.
Function to check if an array is associative:
function isAssociative($array) {
$keys = array_keys($array);
return $keys !== array_keys($keys);
}
This returns true if the array has non-sequential or non-numeric keys.
Examples
-
Sequential:
[ 'apple', 'banana', 'cherry' ]→ keys are 0, 1, 2 -
Sequential (explicit):
[ 0 => 'a', 1 => 'b', 2 => 'c' ] -
Associative:
[ 'name' => 'John', 'age' => 30 ] -
Associative (non-sequential integers):
[ 1 => 'a', 3 => 'b' ]
Note: PHP does not have a built-in function for this, so using array_keys() comparison is the most accurate method.
Basically, just compare the array keys against a generated sequence — if they don't match, it's associative. That’s the most reliable approach.
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