How to check if PHP array is associative or sequential?

Question

You have asked two questions that are not quite equivalent:

• Firstly, how to determine whether an array has only numeric keys
• Secondly, how to determine whether an array has sequential numeric keys, starting from 0

Consider which of these behaviours you actually need. (It may be that either will do for your purposes.)

The first question (simply checking that all keys are numeric) is answered well by Captain kurO.

For the second question (checking whether the array is zero-indexed and sequential), you can use the following function:

function isAssoc(array $arr)
{
    if (array() === $arr) return false;
    return array_keys($arr) !== range(0, count($arr) - 1);
}

var_dump(isAssoc(['a', 'b', 'c'])); // false
var_dump(isAssoc(["0" => 'a', "1" => 'b', "2" => 'c'])); // false
var_dump(isAssoc(["1" => 'a', "0" => 'b', "2" => 'c'])); // true
var_dump(isAssoc(["a" => 'a', "b" => 'b', "c" => 'c'])); // true

To merely check whether the array has non-integer keys (not whether the array is sequentially-indexed or zero-indexed):

function has_string_keys(array $array) {
  return count(array_filter(array_keys($array), 'is_string')) > 0;
}

If there is at least one string key, $array will be regarded as an associative array.

Surely this is a better alternative.

<?php
$arr = array(1,2,3,4);
$isIndexed = array_values($arr) === $arr;

Many commenters in this question don't understand how arrays work in PHP. From the array documentation:

A key may be either an integer or a string. If a key is the standard representation of an integer, it will be interpreted as such (i.e. "8" will be interpreted as 8, while "08" will be interpreted as "08"). Floats in key are truncated to integer. The indexed and associative array types are the same type in PHP, which can both contain integer and string indices.

In other words, there is no such thing as an array key of "8" because it will always be (silently) converted to the integer 8. So trying to differentiate between integers and numeric strings is unnecessary.

If you want the most efficient way to check an array for non-integer keys without making a copy of part of the array (like array_keys() does) or all of it (like foreach does):

function keyedNext( &$arr, &$k){
    $k = key($arr);
    return next($arr);
}

for ($k = key(reset($my_array)); is_int($k); keyedNext($my_array,$k))
    $onlyIntKeys = is_null($k);

This works because key() returns NULL when the current array position is invalid and NULL can never be a valid key (if you try to use NULL as an array key it gets silently converted to "").