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PHP Constants

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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-sensitiv. 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 are:

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:

NameDescription
__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).
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