PHP has eight primitive data types and those are booleans, integer, float, string, array, object, resource, and null. Also, we have to remember that PHP is a weakly typed language and that we are not bothered about the data type declaration while creating those. Though PHP has some static type features, PHP is predominantly a dynamically typed language which means variables are not required to be declared before using it. We can assign a value to a new variable and use it instantly.

For the examples of data structures we have discussed so far can we use any of the primitive data types to represent those structures? Maybe we can or maybe not. Our primitive data types have one particular objective: storing data. In order to achieve some flexibility in performing operations on those data, we will require using the data types in such a way so that we can use them as a particular model and perform some operations. This particular way of handling data through a conceptual model is known as Abstract Data Type, or ADT. ADT also defines a set of possible operations for the data.

We need to understand that ADTs are mainly theoretical concepts which are used in design and analysis of algorithms, data structures, and software design. In contrast, data structures are concrete representations. In order to implement an ADT, we might need to use data types or data structures or both. The most common example of ADTs is stack and queue:

Considering the stack as ADT, it is not only a collection of data but also two important operations called push and pop. Usually, we put a new entry at the top of the stack which is known as push and when we want to take an item, we take from the top which is also known as pop. If we consider PHP array as a stack, we will require additional functionality to achieve these push and pop operations to consider it as stack ADT. Similarly, a queue is also an ADT with two required operations: to add an item at the end of the queue also known as enqueue and remove an item from the beginning of the queue, also known as dequeue. Both sound similar but if we give a close observation we will see that a stack works as a Last-In, First-Out (LIFO) model whereas a queue works as a First-In, First-Out (FIFO) model. These two different mathematical models make them two different ADTs.

Here are some common ADTs:

• List
• Map
• Set
• Stack
• Queue
• Priority queue
• Graph
• Tree

In coming chapters, we will explore more ADTs and implement them as data structures using PHP.