Checking if array is multidimensional or not?

Question

1. What is the most efficient way to check if an array is a flat array of primitive values or if it is a multidimensional array?

Answer 1

is_array($arr[key($arr)]); 

No loops, plain and simple.

Works also with associate arrays not only numeric arrays, which could not contain 0 ( like in the previous example would throw you a warning if the array doesn't have a 0. )