a. Variable name must always begin with a $ symbol.
b. Variable name can never start with a number.
c. Variable names arecase-sensitive

Client server Architecture is classified into three types, as follows
(i) Single Tier Architecture
(ii) Two Tier-Architecture
(iii) N/Mult/Three tire architecture.

A server is a computer or a device that provides functionalities for other programs of devices called "Client".

A server is a computer or a device that provides functionality for other programs or devices, called "clients". This architecture is called the client server model, and a single overall computation is distributed across multiple processes or devices.

Uniform Resource Locater (URL), refers the address of a specific web page or file on the internet.

(i) Resource is a specific variable.
(ii) It hasa reference to an external resource .
(iii) These variables hold specific handlers to handle files and database connections irrespective PHP program.

(i) Open Source
(ii) Case Sensitive
(iii) Simplicity Program language
(iv) Efficiency Program language
(v) Platform Independent Program language
(vi) Security Program language
(vii) Flexibility Program language
(viii) Real-Time Access Monitoring Program language

Operator is a symbol which is used to perform mathematical and logical operations in the programming languages.
Different types of operators in PHP are:
(i) Arithmetic operators
(ii) Assignment operators
(iii) Comparison operators
(iv) Increment/Decrement operators.
(v) Logical operators and
(vi) String operators
1. Arithmetic operators perform arithmetical operations such as addition, subtraction, multiplication and division etc.
2. Assignment operators are performed with numeric values to store a value in a variable. Default assignment operator is "="
3. Comparison operators perform an action to compare two values.
4. Increment/Decrement operators are used to perform increasing (or) decreasing variable's value.
5. Logical operators are used to combine conditional statements.
6. String operators are two operators which are used to perform string related operations such as concatenation and concatenation assignment.

Basis for comparison	Server-side scripting	Client-side scripting
Basic	Works in the back end which could not be visible at the client end.	Works at the front end and script are visible among the users.
Processing	Requires server interaction	Does not need interaction with the server
Languages involved	PHP, ASP.net,Ruby on Rails, ColdFusion, Python, etcetera.	HTML, CSS, JavaScript, etc.
Affect	Could effectively customize the web pages and provide dynamic websites	Can reduce the load to the server.
Server-side scripting	Relatively secure.	Insecure

(i) Server side scripting languages is used in web development.
(ii) It is alternative for web server and itself to deliver a static web page.
(iii) It is used to provide customized interface for the user.
(iv) It also enables the website owner to hide the source code.

Default Syntax:
The default Syntax begins with " < ?php"and closes with "? > ".

Short open Tags
(i) The Short open Tags begins with " < ?" andcloses with "? > " .
(ii) But admin user has to enable Short style tags settings in php.ini file on the server.

HTML Script embed Tags:
HTML Script embed Tags looks just like HTML scripts tags.

An operator is a symbol that performs a given action on a value or group of values. The values the operation is performed upon are known as operands.
The types of operators in PHP are
(i) Arithmetic operators
(ii) Assignment operators
(iii) Comparison operators
(iv) Increment/Decrement operators
(v) Logical operators and
(vi) String operators
1. Arithmetic operators
Arithmetic operators are used to perform math operations, for example, addition, subtraction, multiplication, and division.

Operator	Operator Name	Example	Output
+	Addition	5+2	7
-	Subtraction	5-2	3
*	Multiplication	5*2	10
/	Division	5/2	2.5
%	Modulus	5%2	1

2. Assignment operators
The assignment operator is used to assign a value to a variable. The assignment operator takes the value of its right-hand operand and assigns it to its left-hand operand.

Operator	Operator Name	Example	Output
=	Assignment operator	$a = 5;	5

3. Comparison operators
Comparison operators are used to compare two values and determine whether a certain condition is true or false.

Operator	Operator Name	Example	Output
>	Greater than	5 > 2
<	less than	5<2	False
>=	Greater than or equal to	5>= 2	True
<=	Less than or equal to	5<=2	False
==	Equal to	5==5 5 == 5.0	True False
===	Identical	5===5 5===5.0	True False
!=	Not equal	5!=5 5!=6	True False
!==	Not identical	5!==5 5!==5.0	True False

4.Logical Operators:
Logical operators are used to perform logical operations on variables and values.

Operator	Operator Name	Example	Output
&&	Logical AND	(5>3) && (5>7)	False
II	Logical OR	(5>3) II (5>7)	True
xor	XOR	(5>3) xor (5>7)	True
!	Logical NOT	! (5>3)	False

5. Increment and Decrement operators
In PHP, the increment operator ++ is used to increase the value of a variable by 1, and the decrement operator is used to decrease the value of a variable by 1. The increment and decrement operators can be used as either prefix or postfix operators.

Operator	Name	Output
++ (increment operator)	++ $a Pre-increment	$x = ++ $a
++ (increment operator)	$a + + Post-increment	$x = $a ++
-- (decrement operator)	--$a Pre-decrement	$x = --$a
-- (decrement operator)	$a-- Post-decrement	$x = $a--

6. String operator

String operators have concatenation operator(.) and concatenation assignment operator(-). Concatenation means adding one or more variables to an existing variable.

Operator	Name	Example	Output
. (Dot)	Concatenation	$a = 'Hello'; $b = 'World'; $c= $a. $b;	Hello World
.= (Dot equal to)	Concatenation	$a= 'Hello': $b = 'World'; $a .= $b;	Hello World

Website development has done by the following activities.
1. Website or webpage is developed by the programmer using PHP script.
2. Entire website codes are moved to web server path in a remote server machine.
3. From the client side, the end user open a browser, types the URL of the website (or) webpage and initiates the request to remote server machine over the network.
4. After receiving the request from client machine the web server tries to compile and interpret the PHP code which is available in remote machine.
5. Response will be generated and sent back to the client machine over the network from webserver.
6. Finally the browser is installed in the client machine receives the response and displays the output to user, as shown in fig.

Client side scripting language:
1. Code that is present in a client's HTML page.
2. The language used for client scripting is Java script. It is the most widely used language for client-side scripting.
3. It is compatible and has the ability to run on any browsers.
4. The browser then downloads that code temporarily and processes it without the server.
5. As a result, it is now faster and easier to do client side scripting and leaving less word for the server.
Server side scripting language:
1. Server side scripting usually happens on the back-end of a website.
2. Server side scripting creates a path for the website to the database and all behind the scene work which organizes and runs a website. In server side scripting, the responses can be customized based on requirement of the user.
3. Example of server side programming language is PHP scripting language.
4. It can be executed via an interpreter which is installed in the webservers or CGI (Common Gateway Interface).
5. The most of the webservers such as Apache Tomcat, Microsoft IIS (Internet Information Server) supports the PHP interpreter module.