Magic Constants

Magic constants are predefined constants that change depending on where they are used.

PHP provides several magic constants whose value depends on the context. Their names begin and end with double underscores.

Example

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Log messages that include the file and line they came from (__FILE__, __LINE__).
Framework helpers that report the current class or method (__CLASS__, __METHOD__).
Quick debugging — printing __FUNCTION__ to trace which function produced output.

__LINE__ and __FILE__ resolve to exactly where they appear in the source.

<?php
echo "Error on line " . __LINE__ . "\n";
echo "In file " . __FILE__;

__CLASS__ and __FUNCTION__ describe the current class and method at runtime.

<?php
class User {
    public function whoami() {
        return __CLASS__ . "::" . __FUNCTION__;
    }
}
echo (new User)->whoami(); // User::whoami

__DIR__ is the directory of the current file — ideal for require/include paths.

<?php
// Path relative to THIS file's folder
$path = __DIR__ . "/config.php";
echo $path;