Ans: The array_merge() function merges one or more arrays into one array. This function returns one array with the elements of all the parameter arrays.

The array_slice() function returns selected parts of an array. The array_slice() function returns part of an array as specified by the start and length parameters. If start is a positive number, it means that the ”slice” will start that many elements from the beginning of the array.

Ans: The array_merge() function merges one or more arrays into one array. This function returns one array with the elements of all the parameter arrays.

The array_slice() function returns selected parts of an array. The array_slice() function returns part of an array as specified by the start and length parameters. If start is a positive number, it means that the ”slice” will start that many elements from the beginning of the array.

Abstract class: An Abstract. Class is a class that cannot be instantiated but instead serves as a base class to be inherited by other classes.
Interface: An Interfaces is a collection of unimplemented method definitions and constantswithout specifying exactly how it must be implemented.

An exception is an abnormal condition that arises in a code sequence at run time. Basically there are four important keywords which form the main pillars of exception handling: try, catch, throw and finally. 

The PDO ( PHP Data Objects ) extension defines a lightweight, consistent interface for accessing databases in PHP. if you are using the PDO API, you could switch the database server you used, from say PgSQL to MySQL, and only need to make minor changes to your PHP code.  

While PDO has its advantages, such as a clean, simple, portable API but its main disadvantage is that it doesn't allow you to use all of the advanced features that are available in the latest versions of MySQL server. For example, PDO does not allow you to use MySQL's support for Multiple Statements.