diff --git a/README.md b/README.md
index 5e6d141..7549a64 100644
--- a/README.md
+++ b/README.md
@@ -5,7 +5,6 @@ This reusable package bootstraps our PHPUnit unit and integration tests. It incl
 - bootstrapping for both Unit and Integration tests
 - `phpunit.xml.dist` for each test suite
 - `TestCase` for each test suite
-- Common polyfill functions
 
 This means your repo only needs its tests. w00t!
 
diff --git a/Tests/Fixtures/ApiTrait/getApiCredential.php b/Tests/Fixtures/ApiTrait/getApiCredential.php
new file mode 100644
index 0000000..ab67453
--- /dev/null
+++ b/Tests/Fixtures/ApiTrait/getApiCredential.php
@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+return [
+	'environment variable is set'                => [
+		'env_name'             => 'WPMEDIA_PHPUNIT_TEST_CREDENTIAL',
+		'env_value'            => 'from_env',
+		'config_filename'      => null,
+		'config_file_contents' => null,
+		'credential_name'      => 'WPMEDIA_PHPUNIT_TEST_CREDENTIAL',
+		'expected'             => 'from_env',
+	],
+
+	'no config file is set'                      => [
+		'env_name'             => null,
+		'env_value'            => null,
+		'config_filename'      => null,
+		'config_file_contents' => null,
+		'credential_name'      => 'WPMEDIA_PHPUNIT_TEST_UNSET_CONSTANT',
+		'expected'             => '',
+	],
+
+	'config file is not readable'                => [
+		'env_name'             => null,
+		'env_value'            => null,
+		'config_filename'      => 'missing-credentials.php',
+		'config_file_contents' => null,
+		'credential_name'      => 'WPMEDIA_PHPUNIT_TEST_UNSET_CONSTANT',
+		'expected'             => '',
+	],
+
+	'constant is defined in the config file'     => [
+		'env_name'             => null,
+		'env_value'            => null,
+		'config_filename'      => 'credentials.php',
+		'config_file_contents' => "<?php define( 'WPMEDIA_PHPUNIT_TEST_CONSTANT', 'from_constant' );",
+		'credential_name'      => 'WPMEDIA_PHPUNIT_TEST_CONSTANT',
+		'expected'             => 'from_constant',
+	],
+
+	'constant is not defined in the config file' => [
+		'env_name'             => null,
+		'env_value'            => null,
+		'config_filename'      => 'empty-credentials.php',
+		'config_file_contents' => '<?php // No constants defined here.',
+		'credential_name'      => 'WPMEDIA_PHPUNIT_TEST_UNDEFINED_CONSTANT',
+		'expected'             => '',
+	],
+];
diff --git a/Tests/Unit/ApiTrait/ApiTraitTestDouble.php b/Tests/Unit/ApiTrait/ApiTraitTestDouble.php
new file mode 100644
index 0000000..9f75d6b
--- /dev/null
+++ b/Tests/Unit/ApiTrait/ApiTraitTestDouble.php
@@ -0,0 +1,56 @@
+<?php
+
+declare(strict_types=1);
+
+namespace WPMedia\PHPUnit\Tests\Unit\ApiTrait;
+
+use WPMedia\PHPUnit\Integration\ApiTrait;
+
+/**
+ * Exposes the otherwise protected members of ApiTrait for the tests to drive.
+ */
+class ApiTraitTestDouble {
+	use ApiTrait;
+
+	/**
+	 * Name of the API credentials config file, if applicable.
+	 *
+	 * @var string|null
+	 */
+	protected static $api_credentials_config_file;
+
+	/**
+	 * Points the trait at the config file to load for the current scenario.
+	 *
+	 * @param string $path_to_config Path to the directory holding the config file.
+	 * @param string $config_file    Name of the config file.
+	 *
+	 * @return void
+	 */
+	public static function set_config_file( $path_to_config, $config_file ) {
+		self::pathToApiCredentialsConfigFile( $path_to_config );
+
+		static::$api_credentials_config_file = $config_file;
+	}
+
+	/**
+	 * Gets the credential's value for the given name.
+	 *
+	 * @param string $name Name of the environment variable or constant to find.
+	 *
+	 * @return string returns the value if available; else an empty string.
+	 */
+	public static function get_credential( $name ) {
+		return static::getApiCredential( $name );
+	}
+
+	/**
+	 * Resets the trait's static state so each scenario starts from a clean slate.
+	 *
+	 * @return void
+	 */
+	public static function reset() {
+		self::$path_to_config                = null;
+		static::$api_credentials_config_file = null;
+	}
+}
diff --git a/Tests/Unit/ApiTrait/getApiCredential.php b/Tests/Unit/ApiTrait/getApiCredential.php
new file mode 100644
index 0000000..47b852e
--- /dev/null
+++ b/Tests/Unit/ApiTrait/getApiCredential.php
@@ -0,0 +1,93 @@
+<?php
+
+declare(strict_types=1);
+
+namespace WPMedia\PHPUnit\Tests\Unit\ApiTrait;
+
+use WPMedia\PHPUnit\Unit\TestCase;
+
+/**
+ * Tests WPMedia\PHPUnit\Integration\ApiTrait::getApiCredential().
+ *
+ * @covers WPMedia\PHPUnit\Integration\ApiTrait::getApiCredential
+ * @group  ApiTrait
+ */
+class Test_GetApiCredential extends TestCase {
+
+	/**
+	 * Path to the temporary directory used to hold the config file for the scenarios that need one.
+	 *
+	 * @var string
+	 */
+	private $tmp_dir;
+
+	/**
+	 * Resets the test double and prepares a scratch directory before each scenario.
+	 *
+	 * @return void
+	 */
+	protected function set_up() {
+		parent::set_up();
+
+		ApiTraitTestDouble::reset();
+
+		$this->tmp_dir = sys_get_temp_dir() . '/wpmedia-phpunit-apitrait-' . uniqid();
+
+		mkdir( $this->tmp_dir ); // phpcs:ignore WordPress.WP.AlternativeFunctions.file_system_operations_mkdir -- WP_Filesystem is not available in the Unit test suite; this creates a throwaway scratch directory.
+	}
+
+	/**
+	 * Removes the scratch directory after each scenario.
+	 *
+	 * @return void
+	 */
+	protected function tear_down() {
+		array_map( 'unlink', glob( "{$this->tmp_dir}/*.php" ) );
+		rmdir( $this->tmp_dir ); // phpcs:ignore WordPress.WP.AlternativeFunctions.file_system_operations_rmdir -- WP_Filesystem is not available in the Unit test suite; this removes the throwaway scratch directory.
+
+		parent::tear_down();
+	}
+
+	/**
+	 * Asserts that getApiCredential() returns the expected value for each scenario.
+	 *
+	 * @dataProvider getApiCredentialDataProvider
+	 *
+	 * @param string|null $env_name             Name of the environment variable to set before running the scenario, if any.
+	 * @param string|null $env_value            Value to set the environment variable to, if any.
+	 * @param string|null $config_filename      Name of the config file to point the trait at, if any.
+	 * @param string|null $config_file_contents Contents to write to the config file before running the scenario, if any.
+	 * @param string      $credential_name      Name of the environment variable or constant to look up.
+	 * @param string      $expected             Expected return value.
+	 *
+	 * @return void
+	 */
+	public function testShouldReturnTheExpectedCredential( $env_name, $env_value, $config_filename, $config_file_contents, $credential_name, $expected ) {
+		if ( null !== $env_name ) {
+			putenv( "{$env_name}={$env_value}" ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.runtime_configuration_putenv -- Test-only: simulates the environment variable that getApiCredential() reads via getenv().
+		}
+
+		if ( null !== $config_filename ) {
+			if ( null !== $config_file_contents ) {
+				file_put_contents( $this->tmp_dir . '/' . $config_filename, $config_file_contents ); // phpcs:ignore WordPress.WP.AlternativeFunctions.file_system_operations_file_put_contents -- WP_Filesystem is not available in the Unit test suite; this writes the throwaway config file fixture.
+			}
+
+			ApiTraitTestDouble::set_config_file( $this->tmp_dir . '/', $config_filename );
+		}
+
+		$this->assertSame( $expected, ApiTraitTestDouble::get_credential( $credential_name ) );
+
+		if ( null !== $env_name ) {
+			putenv( $env_name ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.runtime_configuration_putenv -- Test-only: unsets the environment variable set above.
+		}
+	}
+
+	/**
+	 * Provides the scenarios from the Fixtures directory.
+	 *
+	 * @return array test data.
+	 */
+	public function getApiCredentialDataProvider() {
+		return $this->getTestData( __DIR__, 'getApiCredential' );
+	}
+}
diff --git a/phpcs.xml.dist b/phpcs.xml.dist
index 26c2a16..70f138d 100644
--- a/phpcs.xml.dist
+++ b/phpcs.xml.dist
@@ -11,11 +11,17 @@
 	opportunistically, and widen this to the full source tree (src/ plus Tests/) once the
 	codebase is fully compliant.
 	-->
+	<file>src/Integration/ApiTrait.php</file>
 	<file>src/Integration/HttpRequestTrait.php</file>
+	<file>src/TestCaseTrait.php</file>
+	<file>src/Unit/TestCase.php</file>
 	<file>src/VirtualFilesystemTestTrait.php</file>
+	<file>Tests/Fixtures/ApiTrait/getApiCredential.php</file>
+	<file>Tests/Fixtures/VirtualFilesystemTestTrait/getDefaultVfs.php</file>
+	<file>Tests/Unit/ApiTrait/ApiTraitTestDouble.php</file>
+	<file>Tests/Unit/ApiTrait/getApiCredential.php</file>
 	<file>Tests/Unit/VirtualFilesystemDirect/TestCase.php</file>
 	<file>Tests/Unit/VirtualFilesystemTestTrait/getDefaultVfs.php</file>
-	<file>Tests/Fixtures/VirtualFilesystemTestTrait/getDefaultVfs.php</file>
 	<exclude-pattern>vendor/*</exclude-pattern>
 	<arg value="sp"/>
 	<arg name="colors"/>
diff --git a/phpstan-baseline.neon b/phpstan-baseline.neon
index 2a37334..89297ff 100644
--- a/phpstan-baseline.neon
+++ b/phpstan-baseline.neon
@@ -54,12 +54,6 @@ parameters:
 			count: 1
 			path: src/Integration/AdminTestCase.php
 
-		-
-			rawMessage: 'Method WPMedia\PHPUnit\Integration\AjaxTestCase::getApiCredential() should return string but returns bool.'
-			identifier: return.type
-			count: 1
-			path: src/Integration/AjaxTestCase.php
-
 		-
 			rawMessage: 'Class WP_REST_Request referenced with incorrect case: WP_Rest_Request.'
 			identifier: class.nameCase
@@ -84,12 +78,6 @@ parameters:
 			count: 1
 			path: src/Integration/RESTfulTestCase.php
 
-		-
-			rawMessage: 'Method WPMedia\PHPUnit\Integration\RESTfulTestCase::getApiCredential() should return string but returns bool.'
-			identifier: return.type
-			count: 1
-			path: src/Integration/RESTfulTestCase.php
-
 		-
 			rawMessage: Variable $wp_rest_server in PHPDoc tag @var does not exist.
 			identifier: varTag.variableNotFound
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
index 2080dd3..c376acc 100644
--- a/phpstan.neon.dist
+++ b/phpstan.neon.dist
@@ -13,7 +13,6 @@ parameters:
         - src/bootstrap-functions.php
         - src/Integration/
         - src/Unit/
-        - src/Fixtures/
         - Tests/
     scanFiles:
         - vendor/php-stubs/wordpress-stubs/wordpress-stubs.php
diff --git a/src/Fixtures/polyfills.php b/src/Fixtures/polyfills.php
deleted file mode 100644
index 94618a1..0000000
--- a/src/Fixtures/polyfills.php
+++ /dev/null
@@ -1,32 +0,0 @@
-<?php
-
-if ( ! function_exists( 'rocket_has_constant' ) ) {
-	/**
-	 * Checks if the constant is defined.
-	 *
-	 * @param string $constant_name Name of the constant to check.
-	 *
-	 * @return bool true when constant is defined; else, false.
-	 */
-	function rocket_has_constant( $constant_name ) {
-		return defined( $constant_name );
-	}
-}
-
-if ( ! function_exists( 'rocket_get_constant' ) ) {
-	/**
-	 * Gets the constant is defined.
-	 *
-	 * @param string $constant_name Name of the constant to check.
-	 * @param mixed|null $default Optional. Default value to return if constant is not defined.
-	 *
-	 * @return bool true when constant is defined; else, false.
-	 */
-	function rocket_get_constant( $constant_name, $default = null ) {
-		if ( ! rocket_has_constant( $constant_name ) ) {
-			return $default;
-		}
-
-		return constant( $constant_name );
-	}
-}
diff --git a/src/Integration/ApiTrait.php b/src/Integration/ApiTrait.php
index bcf88fb..73c39c2 100644
--- a/src/Integration/ApiTrait.php
+++ b/src/Integration/ApiTrait.php
@@ -1,11 +1,26 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WPMedia\PHPUnit\Integration;
 
 trait ApiTrait {
+
+	/**
+	 * Path to the directory holding the local API credentials config file.
+	 *
+	 * @var string|null
+	 */
 	protected static $path_to_config;
 
-	protected static function pathToApiCredentialsConfigFile( $path ) {
+	/**
+	 * Sets the path to the directory holding the local API credentials config file.
+	 *
+	 * @param string $path Path to the directory holding the config file.
+	 *
+	 * @return void
+	 */
+	protected static function pathToApiCredentialsConfigFile( $path ) { // phpcs:ignore WordPress.NamingConventions.ValidFunctionName.MethodNameInvalid -- Public API method; renaming would be a breaking change for consumers.
 		self::$path_to_config = $path;
 	}
 
@@ -16,7 +31,7 @@ protected static function pathToApiCredentialsConfigFile( $path ) {
 	 *
 	 * @return string returns the value if available; else an empty string.
 	 */
-	protected static function getApiCredential( $name ) {
+	protected static function getApiCredential( $name ) { // phpcs:ignore WordPress.NamingConventions.ValidFunctionName.MethodNameInvalid -- Public API method; renaming would be a breaking change for consumers.
 		$var = getenv( $name );
 		if ( ! empty( $var ) ) {
 			return $var;
@@ -34,6 +49,10 @@ protected static function getApiCredential( $name ) {
 		// This file is local to the developer's machine and not stored in the repo.
 		require_once $config_file;
 
-		return rocket_get_constant( $name, '' );
+		if ( ! defined( $name ) ) {
+			return '';
+		}
+
+		return constant( $name );
 	}
 }
diff --git a/src/TestCaseTrait.php b/src/TestCaseTrait.php
index 25e423c..469f1fd 100644
--- a/src/TestCaseTrait.php
+++ b/src/TestCaseTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WPMedia\PHPUnit;
 
 use ReflectionClass;
@@ -9,10 +11,6 @@
 
 trait TestCaseTrait {
 
-	protected static function stubPolyfills() {
-		require_once __DIR__ . '/Fixtures/polyfills.php';
-	}
-
 	/**
 	 * Gets the test data, if it exists, for this test class.
 	 *
@@ -21,13 +19,13 @@ protected static function stubPolyfills() {
 	 *
 	 * @return array array of test data.
 	 */
-	protected function getTestData( $dir, $filename ) {
+	protected function getTestData( $dir, $filename ) { // phpcs:ignore WordPress.NamingConventions.ValidFunctionName.MethodNameInvalid -- Public API method; renaming would be a breaking change for consumers.
 		if ( empty( $dir ) || empty( $filename ) ) {
 			return [];
 		}
 
-		$dir = str_replace( [ 'Integration', 'Unit' ], 'Fixtures', $dir );
-		$dir = rtrim( $dir, '\\/' );
+		$dir      = str_replace( [ 'Integration', 'Unit' ], 'Fixtures', $dir );
+		$dir      = rtrim( $dir, '\\/' );
 		$testdata = "$dir/{$filename}.php";
 
 		return is_readable( $testdata )
@@ -56,14 +54,14 @@ protected function get_reflective_method( $method_name, $class_name ) {
 	/**
 	 * Get reflective access to the private/protected property.
 	 *
-	 * @param string       $property Property name for which to gain access.
-	 * @param string|mixed $class    Class name or instance.
+	 * @param string       $property   Property name for which to gain access.
+	 * @param string|mixed $class_name Class name or instance.
 	 *
 	 * @return ReflectionProperty|string
 	 * @throws ReflectionException Throws an exception if property does not exist.
 	 */
-	protected function get_reflective_property( $property, $class ) {
-		$class    = new ReflectionClass( $class );
+	protected function get_reflective_property( $property, $class_name ) {
+		$class    = new ReflectionClass( $class_name );
 		$property = $class->getProperty( $property );
 
 		self::set_reflector_accessible( $property, true );
@@ -96,8 +94,18 @@ protected function set_reflective_property( $value, $property, $instance ) {
 		return $property;
 	}
 
-	protected function getNonPublicPropertyValue( $property, $class, $instance = null ) {
-		$property = $this->get_reflective_property( $property, $class );
+	/**
+	 * Gets the value of a private/protected property.
+	 *
+	 * @param string       $property   Property name for which to gain access.
+	 * @param string|mixed $class_name Class name or instance.
+	 * @param mixed|null   $instance   Instance of the target object, if the property is not static.
+	 *
+	 * @return mixed the property's value.
+	 * @throws ReflectionException Throws an exception if property does not exist.
+	 */
+	protected function getNonPublicPropertyValue( $property, $class_name, $instance = null ) { // phpcs:ignore WordPress.NamingConventions.ValidFunctionName.MethodNameInvalid -- Public API method; renaming would be a breaking change for consumers.
+		$property = $this->get_reflective_property( $property, $class_name );
 
 		if ( is_null( $instance ) || $property->isStatic() ) {
 			return $property->getValue();
diff --git a/src/Unit/TestCase.php b/src/Unit/TestCase.php
index 86fd094..53c78bd 100644
--- a/src/Unit/TestCase.php
+++ b/src/Unit/TestCase.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace WPMedia\PHPUnit\Unit;
 
 use WPMedia\PHPUnit\TestCaseTrait;
@@ -7,22 +9,4 @@
 
 abstract class TestCase extends BaseTestCase {
 	use TestCaseTrait;
-
-	/**
-	 * Set to true in root TestCase to stub polyfills in setUpBeforeClass().
-	 *
-	 * @var bool
-	 */
-	protected static $stubPolyfills = false;
-
-	/**
-	 * Prepares the test environment before test class runs.
-	 */
-	public static function setUpBeforeClass(): void {
-		parent::setUpBeforeClass();
-
-		if ( static::$stubPolyfills ) {
-			static::stubPolyfills();
-		}
-	}
 }