PHP Constants

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May 2, 2025 - 10:44
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In PHP, constants are like variables, but once they are defined, they cannot be changed or undefined. They are typically used to store fixed values such as configuration settings, file paths, or magic numbers that shouldn’t be altered during execution.

Defining Constants

There are two main ways to define constants in PHP:

1. define() function

define("SITE_NAME", "My Website"); echo SITE_NAME; // Output: My Website

  • Constant names are case-sensitive by default.

  • By passing true as the third parameter, you can make them case-insensitive: define("SITE_NAME", "My Website", true);

2. const keyword (available from PHP 5.3)

const SITE_URL = "https://example.com"; echo SITE_URL;

  • const must be used at the top-level scope (not inside functions, loops, or if statements).

  • It is always case-sensitive.

Characteristics of Constants

  • They do not start with a $ sign.

  • They are global and can be accessed anywhere in the script.

  • Their value must be a scalar (int, float, string, or boolean) or an array (PHP 5.6+).

Magic Constants

PHP provides a set of predefined constants called magic constants that change depending on where they are used.

Some common ones:

echo __FILE__; // Full path and filename of the file echo __LINE__; // Current line number echo __DIR__; // Directory of the file echo __FUNCTION__; // Function name echo __CLASS__; // Class name