constants in php

By: Unknown on 06:18

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.
constant example:
 <?php 
define("MINSIZE", 50);
 echo MINSIZE;
 echo constant("MINSIZE"); // same thing as the previous line ?> 
Only scalar data (boolean, integer, float and string) can be contained in constants.
Differences between constants and variables  
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.
 Valid and invalid constant names:
 // Valid constant names 
define("ONE", "first thing");
 define("TWO2", "second thing");
 define("THREE_3", "third thing") // 
Invalid constant names 
define("2TWO", "second thing");
 define("__THREE__", "third value");
PHP Magic constants: 
PHP provides a large number of predefined constants to any script which it runs. There are five magical constants that change depending on where they are used. For example, the value of __LINE__ depends on the line that it's used on in your script. These special constants are case-insensitive and are as follows:
 A few "magical" PHP constants ate given below:

 Name                                                             Description

 LINE                       The current line number of the file. __FILE__ The full path and filename of the file.  If used inside an include,the name of the included file is returned. Since PHP 4.0.2, __
FILE                     always contains an absolute path whereas in older versions it contained  relative                                                        path under some circumstances. __
FUNCTION                       The function name. (Added in PHP 4.3.0) As of PHP 5 this constant returns the function name as it was declared (case-sensitive). In PHP 4 its value is always lowercased. 
CLASS                   The class name. (Added in PHP 4.3.0) As of PHP 5 this constant returns the class name as it was declared (case-sensitive). In PHP 4 its value is always lowercased.
 METHOD              The class method name. (Added in PHP 5.0.0) The method name is returned as it was declared (case-sensitive).
 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 
Lets have a look on all operators one by one.
                                                         
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