Use get_defined_constants() to quickly locate constants to define positions (indirect trick)

M66 2025-06-02

We often encounter various predefined constants when developing large projects or maintaining legacy systems using PHP. They may be set by frameworks, libraries, or configuration files, and you often get confused when you debug or refactor your code and want to know where these constants are defined.

PHP provides a built-in function get_defined_constants() that lists all defined constants in the current script. But unfortunately, it does not directly tell you which file or line a constant is defined. This article will introduce a simple indirect technique to help you quickly locate the source of a certain constant using get_defined_constants() .

1. Review of basic usage

get_defined_constants() returns an array containing the names and values of all current constants:

 print_r(get_defined_constants());

The output result is similar:

 Array
(
    [E_ERROR] => 1
    [E_WARNING] => 2
    [MY_CUSTOM_CONSTANT] => abc123
    ...
)

However, this cannot meet the needs of "traceability".

2. Indirect skills: use differential positioning sources of constants

We can use the idea of "difference comparison" to analyze in which file a constant is defined:

The steps are as follows:

Record constant snapshot A : Call get_defined_constants() once before you include the target file.
Include object file : Load files that you suspect have defined constants via require or include .
Record constant snapshot B : After including, call get_defined_constants() again.
Difference analysis : Comparing the differences between snapshots A and B, you can know which constants have been introduced in this file.

Here is a complete example:

 <?php

// first step：Record the initial constant list
$before = get_defined_constants();

// Step 2：Introduce documents you suspect
require_once 'https://m66.net/includes/config.php';

// Step 3：Record the imported constant list
$after = get_defined_constants();

// Step 4：Analyze newly introduced constants
$newConstants = array_diff_assoc($after, $before);

echo "The newly defined constants are：\n";
print_r($newConstants);

After running this code, you will see what new constants are defined in config.php . Although we cannot directly obtain the "definition line number", it can be clearly stated that this file introduces these constants.

3. Advanced usage: precise positioning with debugging tools

If you have confirmed that constants are defined in a file, you can further search for the word define('MY_CONSTANT' quickly with the help of the "Global Search" function in the editor or IDE (such as PhpStorm) to jump to the defined position accurately.

In addition, debuggers such as Xdebug can also monitor the registration of constants in real time after setting breakpoints, but that belongs to a more advanced usage category, and this article will not be expanded for the time being.

4. Summary

Although get_defined_constants() cannot directly tell you "who defined a constant on which line", with the help of the "before and after state comparison", we can quickly determine the source file of the constant. This technique is especially suitable for:

Troubleshooting the source of mysterious constants
Analyze global pollution introduced by external libraries
Monitoring constant leaks in automated testing

You can encapsulate this idea as a general debugging tool, quickly position it when encountering similar problems, and achieve twice the result with half the effort.

get_defined_constants