Though an array is considered to be a data type in PHP, an array is actually a data structure which is mostly used in all programming platforms. In PHP, the array is actually an ordered map (we are going to know about maps after a few more sections). We can store multiple values in a single array as a single variable. Matrix type data are easy to store in an array and hence it is used widely in all programming platforms. Usually arrays are a fixed size collection which is accessed by sequential numeric indexes. In PHP, arrays are implemented differently and you can define dynamic arrays without defining any fixed size of the array. We will explore more about PHP arrays in the next chapter. Arrays can have different dimensions. If an array has only one index to access an element, we call it a single dimension array. But if it requires two or more indexes to access an element, we call it two dimensional or multidimensional arrays respectively. Here are two diagrams of array data structures: