I Present several PHP functions for determining if an array is 'scalar', with differing levels of strictness.

is_scalar_array.php

<?php
/**
 * Since PHP stores all arrays as associative internally, there is no proper
 * definition of a scalar array.
 * 
 * As such, developers are likely to have varying definitions of scalar array,
 * based on their application needs.
 * 
 * In this file, I present 3 increasingly strict methods of determining if an
 * array is scalar.  I feel these methods cover the majority, if not all, of the
 * use cases where a developer would wish to determine the scalar-ness of an array.
 * 
 * @author David Farrell <DavidPFarrell@gmail.com>
 */

/**
 * isArrayWithOnlyIntKeys defines a scalar array as containing
 * only integer keys.
 * 
 * If you are explicitly setting integer keys on an array, you
 * may need this function to determine scalar-ness.
 * 
 * @param array $a
 * @return boolean
 */ 
function isArrayWithOnlyIntKeys(array $a)
{
	if (!is_array($a))
		return false;
	foreach ($a as $k => $v)
		if (!is_int($k))
			return false;
	return true;
}

/**
 * isArrayWithOnlyAscendingIntKeys defines a scalar array as
 * containing only integer keys in ascending (but not necessarily
 * sequential) order.
 * 
 * If you are performing pushes, pops, and unsets on your array,
 * you may need this function to determine scalar-ness.
 * 
 * @param array $a
 * @return boolean
 */ 
function isArrayWithOnlyAscendingIntKeys(array $a)
{
	if (!is_array($a))
		return false;
	$prev = null;
	foreach ($a as $k => $v)
	{
		if (!is_int($k) || (null !== $prev && $k <= $prev))
			return false;
		$prev = $k;
	}
	return true;
}

/**
 * isArrayWithOnlyZeroBasedSequentialIntKeys defines a scalar array
 * as containing only integer keys in sequential, ascending order,
 * starting from 0.
 * 
 * If you are only performing operations on your array that are
 * guaranteed to either maintain consistent key values, or that
 * re-base the keys for consistency, then you can use this function.
 * 
 * @param array $a
 * @return boolean
 */
function isArrayWithOnlyZeroBasedSequentialIntKeys(array $a)
{
	if (!is_array($a))
		return false;
	$i = 0;
	foreach ($a as $k => $v)
		if ($i++ !== $k)
			return false;
	return true;
}