How to Use get_defined_constants Function for Constant Snapshot Comparison in PHPUnit Tests

M66 2025-06-11

In PHP project development, especially when dealing with numerous configuration constants, ensuring that constant values are not accidentally modified during code execution is a critical step. When using PHPUnit for unit testing, the typical approach is to assert function outputs or class behaviors, but detecting changes in constants is not very intuitive.

This article will explain how to use PHP’s built-in get_defined_constants function combined with PHPUnit to implement "snapshot" capturing and comparison of constant sets, making it easy to detect changes in constants during testing.

What is the get_defined_constants function?

get_defined_constants is a PHP function that returns all currently defined constants. It accepts a boolean parameter; if true is passed, it returns an array of constants grouped by category.

Example code:

$constants = get_defined_constants(true);
print_r($constants['user']);  // Retrieves only user-defined constants

This allows us to capture a snapshot of all user-defined constants at the moment, which facilitates subsequent comparisons.

Why perform snapshot comparison of constants?

In unit testing, if a piece of code mistakenly modifies a constant, it can cause unexpected system behavior. Since constants cannot be redefined or changed once set, such "modifications" typically occur by redefining constants with the same name via some workaround or by environment changes causing expected constants to be missing.

By comparing snapshots of constants before and after testing, we can promptly detect unexpected additions, deletions, or changes in constants, ensuring program stability.

Implementation approach

Capture snapshot
Before the test case starts, call get_defined_constants(true)['user'] to save the current array of user-defined constants.
Execute code
Run the code logic under test, which may add or modify constants.
Capture snapshot again
Call get_defined_constants(true)['user'] again to get the current state of constants.
Compare snapshots
Compare the two snapshots to identify newly added, removed, or changed constants.
Assertion verification
Use PHPUnit assertions to ensure constants have not been unexpectedly altered according to expectations.

Example code

Here is an example of using PHPUnit for constant snapshot comparison:

use PHPUnit\Framework\TestCase;
<p>class ConstantSnapshotTest extends TestCase<br>
{<br>
private array $initialConstants;</p>
{
    // Record the initial constant snapshot
    $this->initialConstants = get_defined_constants(true)['user'] ?? [];
}

public function testConstantSnapshot()
{
    // Assume this is the test code block which may define or modify constants
    if (!defined('MY_CONST')) {
        define('MY_CONST', 'initial_value');
    }

    // Re-capture the constant snapshot
    $currentConstants = get_defined_constants(true)['user'] ?? [];

    // Compare the two snapshots to find changes
    $added = array_diff_key($currentConstants, $this->initialConstants);
    $removed = array_diff_key($this->initialConstants, $currentConstants);
    $modified = [];

    foreach ($currentConstants as $key => $value) {
        if (isset($this->initialConstants[$key]) && $this->initialConstants[$key] !== $value) {
            $modified[$key] = [
                'old' => $this->initialConstants[$key],
                'new' => $value,
            ];
        }
    }

    // Output debug information (can be commented out or logged in real tests)
    print_r([
        'added' => $added,
        'removed' => $removed,
        'modified' => $modified,
    ]);

    // Assert no constants have been added or removed
    $this->assertEmpty($added, 'No constants should have been added');
    $this->assertEmpty($removed, 'No constants should have been removed');

    // Assert no constant values have been modified
    $this->assertEmpty($modified, 'No constants should have been modified');
}

}

Notes

Constants cannot be redefined
PHP does not allow redefining constants with the same name unless using special means (such as running in different namespaces). Testing should take this limitation into account.
Environment dependency
Some constants are automatically defined by the environment. Differences in environment before and after tests may cause snapshot discrepancies. It is recommended to compare only the user-defined constants.
Performance impact
Frequent snapshot comparisons involving many constants may slightly affect performance but are generally acceptable.

Applying to real projects

If your project involves complex configurations or dynamically loads modules that define constants at runtime, this snapshot comparison method can automatically detect configuration anomalies within CI pipelines.

For example, during integration testing, it ensures all configuration constants meet expectations, preventing configuration errors during deployment.

Summary

By using the get_defined_constants function along with PHPUnit, you can easily implement constant snapshot comparisons to maintain the stability of constant sets. This approach is simple and effective, suitable for automated testing in PHP projects sensitive to constants.

If you have more advanced needs for managing and testing constants, you can further encapsulate snapshot comparison tools, integrate logging and exception handling to improve code quality and project reliability.

get_defined_constants