As per standard Array definition: An array is a variable which allows you to store multiple value of same Data Type.
In PHP, say
Arrays in PHP are different, there's only one type of array which an store different types. in Your example the items keep their original type. abc is a String, 123 is a number and 4 is a string.
You can even have nonnumeric keys. For Example
$a["a"] = "test".