There are two types of key of array. It can be numerical And associative. 
Numerical : It stores each array element with numeric index name.Numerical keys bear no relation to the value other than the value's position in the array.
Associative : It contains characteristic element or string index name. An associative key logicallybears a direct relation to its corresponding value.

Constructor:- A constructor is defined as a block of code that automatically executes at the time of object instantiation.
Destructor:- We can use destructors to modify the object destruction process. Destructors are created like any other method but must be titled_destruct().

If the try to copy a referenced object. It will simply point back to the addrassing location of the original object To remedy the problems with copying, PHP offers an explicit means for cloning an object is known as object cloning.

Type casting: Converting values from one data type to another is known as type casting.
Type Juggling: Variables are sometimes automatically cast to best fit the circumstances in which they are referenced.

Class: Classes are intended to represent those real-life items that coder like to manipulate within an application.
object: An instance of class is called object.Once the object is created, all characteristic and behaviours define within the class are made available to the newly instantiated object.