REST API: Add wp_is_numeric_array helper function

The API uses this to do special operations on list responses (used
for collections), so we need to detect whether an array is
associative or numeric-indexed.

After much discussion, the bikeshed is to be painted green and gold.

See #33982.


git-svn-id: https://develop.svn.wordpress.org/trunk@34927 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -3176,6 +3176,24 @@ function wp_array_slice_assoc( $array, $keys ) {
 	return $slice;
 }
 
+/**
+ * Determines if the variable is a numeric-indexed array.
+ *
+ * @since 4.4.0
+ *
+ * @param mixed $data Variable to check.
+ * @return bool Whether the variable is a list.
+ */
+function wp_is_numeric_array( $data ) {
+	if ( ! is_array( $data ) ) {
+		return false;
+	}
+
+	$keys = array_keys( $data );
+	$string_keys = array_filter( $keys, 'is_string' );
+	return count( $string_keys ) === 0;
+}
+
 /**
  * Filters a list of objects, based on a set of key => value arguments.
  *