PHP (HYPERTEXT PREPROCESSOR)

                                    PHP
               (HYPERTEXT PREPROCESSOR) 

The PHP Hypertext Pre-processor (PHP) is a programming language that allows web developers to create dynamic content that interacts with databases. PHP is basically used for developing web based software applications. 

PHP started out as a small open source project that evolved as more and more people found out how useful it was. Rasmus Lerdorf unleashed the first version of PHP way back in 1994.  PHP is a recursive acronym for "PHP: Hypertext Preprocessor".  PHP is a server side scripting language that is embedded in HTML. It is used to manage dynamic content, databases, session tracking, even build entire e-commerce sites.  It is integrated with a number of popular databases, including MySQL, PostgreSQL, Oracle, Sybase, Informix, and Microsoft SQL Server.  PHP is pleasingly zippy in its execution, especially when compiled as an Apache module on the Unix side. The MySQL server, once started, executes even very complex queries with huge result sets in record-setting time.  PHP supports a large number of major protocols such as POP3, IMAP, and LDAP. PHP4 added support for Java and distributed object architectures (COM and CORBA), making n-tier development a possibility for the first time.  PHP is forgiving: PHP language tries to be as forgiving as possible.  PHP Syntax is C-Like. Common uses of PHP:  PHP performs system functions, i.e. from files on a system it can create, open, read, write, and close them.  PHP can handle forms, i.e. gather data from files, save data to a file, thru email you can send data, return data to the user.  You add, delete, modify elements within your database thru PHP.  Access cookies variables and set cookies.  Using PHP, you can restrict users to access some pages of your website.  It can encrypt data.

Characteristics of PHP Five important 

characteristics make PHP's practical nature possible: 
 Simplicity 
 Efficiency  
 Security 
 Flexibility 
 Familiarity. 

PHP Environment Setup In order to de velop and run PHP Web pages three vital components need to be installed on your computer system. 

Web Server 
PHP will work with virtually all Web Server software, including Microsoft's Internet Information Server (IIS) but then most often used is freely a Server.  

Database PHP 
will work with virtually all database software, including Oracle and Sybase but most commonly used is freely available MySQL database.

PHP Parser
In order to process PHP script instructions a parser must be in stalled to generate HTML output that can be sent to the Web Browser. This tutorial will guide you how to install PHP parser on your computer.

                  PHP Variable Types 

The main way to store information in the middle of a PHP program is by using a variable. Here are the most important things to know about variables in PHP.  

All variables in PHP are denoted with a leading dollar sign ($).  

The value of a variable is the value of its most recent assignment.  

Variables are assigned with the = operator, with the variable on the left-hand side and the expression to be evaluated on the right. 
 Variables can, but do not need, to be declared before assignment.  

Variables in PHP do not have intrinsic types - a variable does not know in advance whether it will be used to store a number or a string of characters.  

Variables used before they are assigned have default values.  

PHP does a good job of automatically converting types from one to another when necessary.  

PHP variables are Perl-like. 

Interpreting other types as Booleans: 

 Here are the rules for determine the "truth" of any value not already of the Boolean type:  

If the value is a number, it is false if exactly equal to zero and true otherwise.  

If the value is a string, it is false if the string is empty (has zero characters) or is the string "0", and is true otherwise.  

Values of type NULL are always false.  If the value is an array, it is false if it contains no other values, and it is true otherwise. 

For an object, containing a value means having a member variable that has been assigned a value.  

Valid resources are true (although some functions that return resources when they are successful will return FALSE when unsuccessful).  

Don't use double as Booleans. 

                   PHP Constants

A constant is a name or an identifier for a simple value. A constant value cannot change during the execution of the script. By default a constant is case-sensitive. By convention, constant identifiers are always uppercase. A constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. 
If you have defined a constant, it can never be changed or undefined. To define a constant you have to use define() function and to retrieve the value of a constant, you have to simply specifying its name. Unlike with variables, you do not need to have a constant with a $. You can also use the function constant() to read a constant's value if you wish to obtain the constant's name dynamically.

constant() function: 

As indicated by the name, this function will return the value of the constant. This is useful when you want to retrieve value of a constant, but you do not know its name, i.e. It is stored in a variable or returned by a function. 

Differences between constants and variables are:  

There is no need to write a dollar sign ($) before a constant, where as in Variable one has to write a dollar sign.  

Constants cannot be defined by simple assignment, they may only be defined using the define() function. 

 Constants may be defined and accessed anywhere without regard to variable scoping rules.  

Once the Constants have been set, may not be redefined or undefined. 

                 PHP Operator Types

What is Operator? Simple answer can be given using expression 4 + 5 is equal to 9. Here 4 and 5 are called operands and + is called operator. PHP language supports following type of operators.  

Arithmetic Operators  
Comparision Operators  
Logical (or Relational) Operators  Assignment Operators  
Conditional (or ternary) Operators 

                       PHP Loop Types 

Loops in PHP are used to execute the same block of code a specified number of times. PHP supports following four loop types.  

for - loops through a block of code a specified number of times.  

while - loops through a block of code if and as long as a specified condition is true. 

 do...while - loops through a block of code once, and then repeats the loop as long as a special condition is trur.  

foreach - loops through a block of code for each element in an array. 

                The while loop statement

The while statement will execute a block of code if and as long as a test expression is true. If the test expression is true then the code block will be executed. After the code has executed the test expression will again be evaluated and the loop will continue until the test expression is found to be false. 

                     The break statement

The PHP break keyword is used to terminate the execution of a loop prematurely. The break statement is situated inside the statement block. If gives you full control and whenever you want to exit from the loop you can come out. After coming out of a loop immediate statement to the loop will be executed. 

                 The continue statement

The PHP continue keyword is used to halt the current iteration of a loop but it does not terminate the loop. Just like the break statement the continue statement is situated inside the statement block containing the code that the loop executes, preceded by a conditional test. For the pass encountering continue statement, rest of the loop code is skipped and next pass starts.