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PHP Constants

PHP Constants

Hello folks! welcome back to a new edition of our tutorial on PHP. In this tutorial guide, we are going to be studying about the PHP Constants.

PHP constant is a name or an identifier for a simple value. A constant value cannot be changed during the execution of the script. By default, a constant is case-sensitive. By convention, constant identifiers are always uppercase. The name of a constant begins 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.

How to Define a Constant?

To define a constant, you have to make use of define() function and to retrieve the value of a constant, you have to simply specifying its name. Unlike with the variables, you don't need to get a constant with $. You can also make use of the constant() function to read a constant's value if you desire to obtain the constant's name in a dynamic manner.

PHP constant() Function

PHP Constant() Function

As indicated by the name, this function will return the value of the constant.

This is useful when you want to retrieve the value of a constant, but you do not know its name, i.e. it is stored in a variable.


The following below is a simple example -

   define("MINSIZE", 50);
   echo MINSIZE;
   echo constant("MINSIZE"); // same thing as the previous line

Only scalar data (integer, string, boolean and float) can be held in constant.

Difference between Variables & Constants

Difference between constants and variables are -

  • There is no need to write a dollar sign ($) before writing a constant, whereas one needs to write a dollar($) sign in a variable.
  • Constants can't be defined by simple assignment, they can only be defined by using the define() function.
  • Constants may be defined and then accessed anywhere within a program without regarding the variable scoping rules.
  • Once a constants have been set, they may not be redefined or undefined.

Valid and Invalid Constant Names

Following is an example of 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 Magic Constants

PHP provides a large number of predefined constants to any script which it runs.

PHP makes available five magic constants that changes depending on where they are used. For example, the value of __LINE__ is dependent on the line it is used on in your script.

This special constants are case-insensitive and are as follows -

Sr.NoName & Description


The current line number of the 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.



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.



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.



The class method name. (Added in PHP 5.0.0) The method name is returned as it was declared (case-sensitive).

Alright guys! This is where we are rounding up for this tutorial post. In our next tutorial guide, we are going to be discussing about the PHP Operators.

Feel free to ask your questions where necessary and we will attend to them as soon as possible. If this tutorial was helpful to you, you can use the share button to share this tutorial.

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