Explain fork() system call? 

Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel. By default, a Linux user falls under same as userid (UPG) group.

A computer program is a collection of instructions that performs a specific task when executed by a computer

A bootstrap is the process of starting up a computer. It also refers to the program that initializes the operating system (OS) during start-up.

It referred to a bootstrap load button that was used to initiate a hardwired bootstrap program, or smaller program that executed a larger program such as the OS.

C++ is not an Operating System.

An Operating System is a collection of system programs that manages all the other programs application programs in a computer as well as the allocation and use of hardware resources such as the CPU, Memory and the Hard Disk Drive.

It acts as an interface between the hardware and the user level program.

It controls and facilitates the overall operation of a computer. 

Here

UNIX : UNIX is a popular multi-user, multitasking operating system (OS) developed at Bell Labs.

Windows NT : Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993.

DOS : DOS is Disk Operating System. The term DOS can refer to any operating system.

C++ : C++ is a programming language and computing platform.

Hence, C++ is not an Operating System.

soundex()
The soundex() function calculates the soundex key of a string. A soundex key is a four character long alphanumeric string that represent English pronunciation of a word. he soundex() function can be used for spelling applications.
< ?php
$str = "hello";
echo soundex($str);
? >

metaphone()
The metaphone() function calculates the metaphone key of a string. A metaphone key represents how a string sounds if said by an English speaking person. The metaphone() function can be used for spelling applications.
< ?php
echo metaphone("world");
? >