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 $. You can also use the function constant() to read a constant's value if you wish to obtain the constant's name dynamically.
constant() function:
As indicated by the name, this function will return the value of the constant.
This is useful when you want to retrieve value of a constant, but you do not know its name, i.e. It is stored in a variable or returned by a function.
constant() example:
<?php
define("MINSIZE", 50);
echo MINSIZE;
echo constant("MINSIZE"); // same thing as the previous line
?>
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Only scalar data (boolean, integer, float and string) can be contained in constants.
Differences between constants and variables are:
- There is no need to write a dollar sign ($) 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")
// Invalid constant names
define("2TWO", "second thing");
define("__THREE__", "third value");
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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). |
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