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PHP variable and constant

What is variable? 
A variable is a representation of a value. Let's take for instance, A=40; the representation of 40 is A, so A is called the variable. For storing information,  a variable can be called "containers".

Rules for variable 
1. Add a $ (dollar sign) in front or before the variable name.
For example: $No_of_oranges
2. The PHP cannot begin with numerals; despite the fact of adding the dollar sign ($) in front of the variable name. The variable name must begin with alphabets or underscore sign(_).
3. The PHP variable may contain the following characters:

  • Numerals: (0-9)
  • Alphabets: (A-Z) or (a-z) 
  • Underscore: (_)
4. PHP variables are case sensitive. ie. $No_of_oranges and $NO_OF_ORANGES are different variable
5. Variable name can be any length long.

Variable Types:
1. Scalar types:

  • Boolean
  • Integer
  • Float
  • String
2. Compound types:

  • Object
  • Array
3. Special types:

  • Resource 
  • Null
Let's take an example:
<?php
$string_value="hello simply lecture! ";
$integer=126;
$float=3.658;
$boolean=true;
$array=array(1,2,3);
$object=new subject_name();
$null_variable=NULL;
?>

Constants 
PHP constants are similar to variable but they cannot be changed when the script is executed.
The basic features of a constant that distinguish them from a variable are:

  • Constants don't require the $ (dollar sign) before their names 
  • Constants are usually uppercase that distinguish them from normal variable
  • define() is used to define a constant, it requires the name and the value of the name.
Example:
 define("subject name","simply lecture");

  • You can check if a given constant name exist or not.
 Example:
if (defined("subject_name"))
echo "constant subject_name exist";

Pre-defined variables
Pre-defined variables are already installed or defined variables, and PHP provides a large number of pre-defined variables to its users. This pre-defined variables are called "super globals".
The pre-defined variables are given below:

  • $GLOBALS - All variables available in global scope.
  • $_SERVER - Server and execution environment information.
  • $_GET - HTTP GET variable.
  • $_POST - HTTP POST variable.
  • $_FILES - HTTP File upload variable. 
  • $_SESSION - Session variable.
  • $_REQUEST - HTTP Request Variable.
  • $_ENV - Environment variable.
  • $_HTTP COOKIES - HTTP cookies.




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