{"id":57,"date":"2021-03-08T00:05:11","date_gmt":"2021-03-08T00:05:11","guid":{"rendered":"https:\/\/phptutorial.net\/?page_id=57"},"modified":"2025-04-05T14:07:42","modified_gmt":"2025-04-05T14:07:42","slug":"php-constants","status":"publish","type":"page","link":"https:\/\/www.phptutorial.net\/php-tutorial\/php-constants\/","title":{"rendered":"PHP Constants"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you learn about PHP constants and how to use the define()<\/code> function and const<\/code> keyword to define constants.<\/p>\n\n\n\n

Introduction to PHP constants #<\/a><\/h2>\n\n\n\n

A constant is simply a name that holds a single value. As its name implies, the value of a constant cannot be changed during the execution of the PHP script.<\/p>\n\n\n\n

To define a constant, you use the  define()<\/code> function. The  define()<\/code> function takes the constant’s name as the first argument and the constant value as the second. For example:<\/p>\n\n\n

<?php<\/span> \n\ndefine('WIDTH'<\/span>,'1140px'<\/span>);\necho<\/span> WIDTH;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
By convention, constant names are uppercase. Unlike a variable<\/a>, the constant name doesn’t start with the dollar sign($<\/code>).<\/p>\n\n\n\n
By default, constant names are case-sensitive. It means that WIDTH<\/code> and width<\/code> are different constants.<\/p>\n\n\n\n
It’s possible to define case-insensitive constants. However, it’s been deprecated since PHP 7.3<\/p>\n\n\n\n
In PHP 5, a constant can hold a simple value like a number<\/a>, a string<\/a>, or a boolean <\/a>value. From PHP 7.0, a constant can hold an array<\/a>. For example:<\/p>\n\n\n
<?php<\/span>\n\ndefine( 'ORIGIN'<\/span>, [0<\/span>, 0<\/span>] );<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Like superglobal variables, you can access constants from anywhere in the script.<\/p>\n\n\n\n
The const keyword #<\/a><\/h2>\n\n\n\n
PHP provides you with another way to define a constant via the const<\/code> keyword. Here’s the syntax:<\/p>\n\n\n
const<\/span> CONSTANT_NAME = value;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
In this syntax, you define the constant name after the const<\/code> keyword. You use the assignment operator (=) and the constant value to assign a value to a constant. The constant value can be scalar, e.g., a number, a string, or an array.<\/p>\n\n\n\n
The following example uses the const<\/code> keyword to define the SALES_TAX<\/code> constant:<\/p>\n\n\n
<?php<\/span>\n\nconst<\/span> SALES_TAX = 0.085<\/span>;\n\n$gross_price = 100<\/span>;\n\n$net_price = $gross_price * (1<\/span> + SALES_TAX);\n\necho<\/span> $net_price; \/\/ 108.5<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
The following example uses the const<\/code> keyword to define the RGB constant that holds an array:<\/p>\n\n\n
<?php<\/span>\n\nconst<\/span> RGB = ['red'<\/span>, 'green'<\/span>, 'blue'<\/span>];<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
define vs. const #<\/a><\/h2>\n\n\n\n
First, the define()<\/code> is a function while the const<\/code> is a language construct.<\/p>\n\n\n\n
It means that the define()<\/code> function defines a constant at run-time, whereas the const<\/code> keyword defines a constant at compile time.<\/p>\n\n\n\n
In other words, you can use the define()<\/code> function to define a constant conditionally like this:<\/p>\n\n\n
<?php<\/span>\n\nif<\/span>(condition) \n{\n    define('WIDTH'<\/span>, '1140px'<\/span>);\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
However, you cannot use the const<\/code> keyword to define a constant this way. For example, the syntax of the following code is invalid:<\/p>\n\n\n
<?php<\/span> \n\nif<\/span>(condition) \n{\n   const<\/span> WIDTH = '1140px'<\/span>;\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, the define()<\/code> function allows you to define a constant with the name that comes from an expression. For example, the following defines three constants OPTION_1<\/code>, OPTION_2<\/code>, and OPTION_3<\/code> with the values 1, 2, and 3.<\/p>\n\n\n
<?php<\/span>\n\ndefine('PREFIX'<\/span>, 'OPTION'<\/span>);\n\n\ndefine(PREFIX . '_1'<\/span>, 1<\/span>);\ndefine(PREFIX . '_2'<\/span>, 2<\/span>);\ndefine(PREFIX . '_3'<\/span>, 3<\/span>);<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
However, you cannot use the const<\/code> keyword to define a constant name derived from an expression.<\/p>\n\n\n\n
Unless you want to define a constant conditionally or use an expression, you can use the const<\/code> keyword to define constants to clarify the code.<\/p>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
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  • A constant is a name that holds a simple value that cannot be changed during the execution of the script. From PHP 7, a constant can hold an array.<\/li>\n\n\n\n
  • A constant can be accessed from anywhere in the script.<\/li>\n\n\n\n
  • Use the define()<\/code> function or const<\/code> keyword to define a constant.<\/li>\n\n\n\n
  • Use the define()<\/code> function if you want to define a constant conditionally or using an expression.<\/li>\n<\/ul>\n
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    Did you find this tutorial useful?<\/div>\n\t\t
    \n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t Yes <\/span>\n\t\t\t<\/button>\n\n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t No <\/span>\n\t\t\t<\/button>\n\t\t<\/div>\n\t<\/header>\n\n\t
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    <\/div>\n\t\t\t\n\t\t\t
Note that you can use the const<\/code> keyword to define constants inside classes<\/a>.<\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n