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PHP Constant
Define constant- A constant is a variable whose value cannot be changed at runtime.
Suppose we are developing a program that uses the value of PI 3.14, we can use a constant to store its value.
Let’s now look at an example that defines a constant. define('PI',3.14); //creates a constant with a value of 3.14 Once you define PI as 3.14 , writing a code like below will generate an error PI = 4; //PI has been defined as a constant therefore assigning a value is not permissible.
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 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.
Valid and invalid constant names
// Valid constant names
define("ONE", "first thing");
define("TWO2", "second thing");
define("THREE_3", "third thing");
define("__THREE__", "third value");
// Invalid constant names
define("2TWO", "second thing");
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 are given below −
Sr.No Name & Description
1 _LINE_
The current line number of the file.
2 _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.
3 _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.
4 _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.
5 _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
• Comparison Operators
• Logical (or Relational) Operators
• Assignment Operators
• Conditional (or ternary) Operators