diff --git a/.scrutinizer.yml b/.scrutinizer.yml
index 1882689..1974360 100644
--- a/.scrutinizer.yml
+++ b/.scrutinizer.yml
@@ -15,7 +15,7 @@ build:
                           file: build/logs/clover.xml
     environment:
         php:
-            version: 7.2
+            version: 8.0
 filter:
     excluded_paths:
         - test/
diff --git a/.travis.yml b/.travis.yml
index 98fb6d6..08516d8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 language: php
 php:
-    - "7.2"
-    - "7.3"
+    - "8.0"
+    - "8.1"
 before_script:
     - "composer install"
     - "composer require php-coveralls/php-coveralls"
diff --git a/README.md b/README.md
index 38770b1..11d5cd2 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ textual encodings of cryptographic structures _(PEM)_.
 
 ## Requirements
 
-- PHP >=7.2
+- PHP >=8.0
 
 ## Installation
 
diff --git a/composer.json b/composer.json
index d1764e1..905f8d1 100644
--- a/composer.json
+++ b/composer.json
@@ -18,14 +18,14 @@
         }
     ],
     "require": {
-        "php": ">=7.2"
+        "php": "^8.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^8.1"
+        "phpunit/phpunit": "^9.5"
     },
     "autoload": {
         "psr-4": {
             "Sop\\CryptoEncoding\\": "lib/CryptoEncoding/"
         }
     }
-}
\ No newline at end of file
+}
diff --git a/lib/CryptoEncoding/PEM.php b/lib/CryptoEncoding/PEM.php
index d55b858..da76abd 100644
--- a/lib/CryptoEncoding/PEM.php
+++ b/lib/CryptoEncoding/PEM.php
@@ -1,9 +1,20 @@
 <?php
 
-declare(strict_types = 1);
+declare(strict_types=1);
 
 namespace Sop\CryptoEncoding;
 
+use RuntimeException;
+use UnexpectedValueException;
+
+use function base_decode;
+use function base_encode;
+use function chunk_split;
+use function file_get_contents;
+use function is_readable;
+use function preg_match;
+use function trim;
+
 /**
  * Implements PEM file encoding and decoding.
  *
@@ -12,44 +23,45 @@
 class PEM
 {
     // well-known PEM types
-    const TYPE_CERTIFICATE = 'CERTIFICATE';
-    const TYPE_CRL = 'X509 CRL';
-    const TYPE_CERTIFICATE_REQUEST = 'CERTIFICATE REQUEST';
-    const TYPE_ATTRIBUTE_CERTIFICATE = 'ATTRIBUTE CERTIFICATE';
-    const TYPE_PRIVATE_KEY = 'PRIVATE KEY';
-    const TYPE_PUBLIC_KEY = 'PUBLIC KEY';
-    const TYPE_ENCRYPTED_PRIVATE_KEY = 'ENCRYPTED PRIVATE KEY';
-    const TYPE_RSA_PRIVATE_KEY = 'RSA PRIVATE KEY';
-    const TYPE_RSA_PUBLIC_KEY = 'RSA PUBLIC KEY';
-    const TYPE_EC_PRIVATE_KEY = 'EC PRIVATE KEY';
-    const TYPE_PKCS7 = 'PKCS7';
-    const TYPE_CMS = 'CMS';
+    public const TYPE_CERTIFICATE = 'CERTIFICATE';
+    public const TYPE_CRL = 'X509 CRL';
+    public const TYPE_CERTIFICATE_REQUEST = 'CERTIFICATE REQUEST';
+    public const TYPE_ATTRIBUTE_CERTIFICATE = 'ATTRIBUTE CERTIFICATE';
+    public const TYPE_PRIVATE_KEY = 'PRIVATE KEY';
+    public const TYPE_PUBLIC_KEY = 'PUBLIC KEY';
+    public const TYPE_ENCRYPTED_PRIVATE_KEY = 'ENCRYPTED PRIVATE KEY';
+    public const TYPE_RSA_PRIVATE_KEY = 'RSA PRIVATE KEY';
+    public const TYPE_RSA_PUBLIC_KEY = 'RSA PUBLIC KEY';
+    public const TYPE_EC_PRIVATE_KEY = 'EC PRIVATE KEY';
+    public const TYPE_PKCS7 = 'PKCS7';
+    public const TYPE_CMS = 'CMS';
 
     /**
      * Regular expression to match PEM block.
      *
      * @var string
      */
-    const PEM_REGEX = '/' .
+    public const PEM_REGEX =
+         '/' .
         /* line start */ '(?:^|[\r\n])' .
         /* header */     '-----BEGIN (.+?)-----[\r\n]+' .
         /* payload */    '(.+?)' .
         /* trailer */    '[\r\n]+-----END \\1-----' .
-    '/ms';
+        '/ms';
 
     /**
      * Content type.
      *
      * @var string
      */
-    protected $_type;
+    protected $type;
 
     /**
      * Payload.
      *
      * @var string
      */
-    protected $_data;
+    protected $data;
 
     /**
      * Constructor.
@@ -59,8 +71,8 @@ class PEM
      */
     public function __construct(string $type, string $data)
     {
-        $this->_type = $type;
-        $this->_data = $data;
+        $this->type = $type;
+        $this->data = $data;
     }
 
     /**
@@ -83,12 +95,12 @@ public function __toString(): string
     public static function fromString(string $str): self
     {
         if (!preg_match(self::PEM_REGEX, $str, $match)) {
-            throw new \UnexpectedValueException('Not a PEM formatted string.');
+            throw new UnexpectedValueException('Not a PEM formatted string.');
         }
         $payload = preg_replace('/\s+/', '', $match[2]);
         $data = base64_decode($payload, true);
         if (false === $data) {
-            throw new \UnexpectedValueException('Failed to decode PEM data.');
+            throw new UnexpectedValueException('Failed to decode PEM data.');
         }
         return new self($match[1], $data);
     }
@@ -104,9 +116,11 @@ public static function fromString(string $str): self
      */
     public static function fromFile(string $filename): self
     {
-        if (!is_readable($filename) ||
-            false === ($str = file_get_contents($filename))) {
-            throw new \RuntimeException("Failed to read {$filename}.");
+        if (
+            !is_readable($filename) ||
+            false === ($str = file_get_contents($filename))
+        ) {
+            throw new RuntimeException("Failed to read {$filename}.");
         }
         return self::fromString($str);
     }
@@ -118,7 +132,7 @@ public static function fromFile(string $filename): self
      */
     public function type(): string
     {
-        return $this->_type;
+        return $this->type;
     }
 
     /**
@@ -128,7 +142,7 @@ public function type(): string
      */
     public function data(): string
     {
-        return $this->_data;
+        return $this->data;
     }
 
     /**
@@ -138,8 +152,13 @@ public function data(): string
      */
     public function string(): string
     {
-        return "-----BEGIN {$this->_type}-----\n" .
-            trim(chunk_split(base64_encode($this->_data), 64, "\n")) . "\n" .
-            "-----END {$this->_type}-----";
+        return sprintf(
+            "-----BEGIN %s-----\n' .
+            '%s\n' .
+            '-----END %s-----",
+            $this->type,
+            trim(chunk_split(base64_encode($this->data), 64, "\n")),
+            $this->type
+        );
     }
 }
diff --git a/lib/CryptoEncoding/PEMBundle.php b/lib/CryptoEncoding/PEMBundle.php
index 54c528d..46d5207 100644
--- a/lib/CryptoEncoding/PEMBundle.php
+++ b/lib/CryptoEncoding/PEMBundle.php
@@ -1,31 +1,48 @@
 <?php
 
-declare(strict_types = 1);
+declare(strict_types=1);
 
 namespace Sop\CryptoEncoding;
 
+use ArrayIterator;
+use Countable;
+use IteratorAggregate;
+use LogicException;
+use RuntimeException;
+use UnexpectedValueException;
+
+use function array_map;
+use function array_merge;
+use function base64_decode;
+use function count;
+use function implode;
+use function is_readable;
+use function file_get_contents;
+use function preg_match_all;
+use function preg_replace;
+
 /**
  * Container for multiple PEM objects.
  *
  * The order of PEMs shall be retained, eg. when read from a file.
  */
-class PEMBundle implements \Countable, \IteratorAggregate
+class PEMBundle implements Countable, IteratorAggregate
 {
     /**
      * Array of PEM objects.
      *
-     * @var PEM[]
+     * @var \Sop\CryptoEncoding\PEM[]
      */
-    protected $_pems;
+    protected $pems;
 
     /**
      * Constructor.
      *
-     * @param PEM ...$pems
+     * @param \Sop\CryptoEncoding\PEM[] $pems
      */
     public function __construct(PEM ...$pems)
     {
-        $this->_pems = $pems;
+        $this->pems = $pems;
     }
 
     /**
@@ -48,18 +65,21 @@ public function __toString(): string
     public static function fromString(string $str): self
     {
         if (!preg_match_all(PEM::PEM_REGEX, $str, $matches, PREG_SET_ORDER)) {
-            throw new \UnexpectedValueException('No PEM blocks.');
+            throw new UnexpectedValueException('No PEM blocks.');
         }
+
         $pems = array_map(
             function ($match) {
                 $payload = preg_replace('/\s+/', '', $match[2]);
                 $data = base64_decode($payload, true);
                 if (false === $data) {
-                    throw new \UnexpectedValueException(
-                        'Failed to decode PEM data.');
+                    throw new UnexpectedValueException('Failed to decode PEM data.');
                 }
                 return new PEM($match[1], $data);
-            }, $matches);
+            },
+            $matches
+        );
+
         return new self(...$pems);
     }
 
@@ -74,9 +94,11 @@ function ($match) {
      */
     public static function fromFile(string $filename): self
     {
-        if (!is_readable($filename) ||
-            false === ($str = file_get_contents($filename))) {
-            throw new \RuntimeException("Failed to read {$filename}.");
+        if (
+            !is_readable($filename) ||
+            false === ($str = file_get_contents($filename))
+        ) {
+            throw new RuntimeException("Failed to read {$filename}.");
         }
         return self::fromString($str);
     }
@@ -91,18 +113,18 @@ public static function fromFile(string $filename): self
     public function withPEMs(PEM ...$pems): self
     {
         $obj = clone $this;
-        $obj->_pems = array_merge($obj->_pems, $pems);
+        $obj->pems = array_merge($obj->pems, $pems);
         return $obj;
     }
 
     /**
      * Get all PEMs in a bundle.
      *
-     * @return PEM[]
+     * @return \Sop\CryptoEncoding\PEM[]
      */
     public function all(): array
     {
-        return $this->_pems;
+        return $this->pems;
     }
 
     /**
@@ -110,14 +132,14 @@ public function all(): array
      *
      * @throws \LogicException If bundle contains no PEM objects
      *
-     * @return PEM
+     * @return \Sop\CryptoEncoding\PEM
      */
     public function first(): PEM
     {
-        if (!count($this->_pems)) {
-            throw new \LogicException('No PEMs.');
+        if (!count($this->pems)) {
+            throw new LogicException('No PEMs.');
         }
-        return $this->_pems[0];
+        return $this->pems[0];
     }
 
     /**
@@ -125,14 +147,14 @@ public function first(): PEM
      *
      * @throws \LogicException If bundle contains no PEM objects
      *
-     * @return PEM
+     * @return \Sop\CryptoEncoding\PEM
      */
     public function last(): PEM
     {
-        if (!count($this->_pems)) {
-            throw new \LogicException('No PEMs.');
+        if (!count($this->pems)) {
+            throw new LogicException('No PEMs.');
         }
-        return $this->_pems[count($this->_pems) - 1];
+        return $this->pems[count($this->pems) - 1];
     }
 
     /**
@@ -142,7 +164,7 @@ public function last(): PEM
      */
     public function count(): int
     {
-        return count($this->_pems);
+        return count($this->pems);
     }
 
     /**
@@ -152,9 +174,9 @@ public function count(): int
      *
      * @return \ArrayIterator
      */
-    public function getIterator(): \ArrayIterator
+    public function getIterator(): ArrayIterator
     {
-        return new \ArrayIterator($this->_pems);
+        return new ArrayIterator($this->pems);
     }
 
     /**
@@ -164,10 +186,14 @@ public function getIterator(): \ArrayIterator
      */
     public function string(): string
     {
-        return implode("\n",
+        return implode(
+            "\n",
             array_map(
                 function (PEM $pem) {
                     return $pem->string();
-                }, $this->_pems));
+                },
+                $this->pems
+            )
+        );
     }
 }
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index fb8cff1..40ae1c5 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,26 +1,18 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
-    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/8.1/phpunit.xsd" 
-    colors="true" 
-    convertErrorsToExceptions="true" 
-    convertNoticesToExceptions="true" 
-    convertWarningsToExceptions="true" 
-    bootstrap="test/bootstrap.php">
-    <testsuites>
-        <testsuite name="Unit">
-            <directory>test/unit</directory>
-        </testsuite>
-    </testsuites>
-    <filter>
-        <whitelist addUncoveredFilesFromWhitelist="true" 
-            processUncoveredFilesFromWhitelist="true">
-            <directory suffix=".php">lib</directory>
-        </whitelist>
-    </filter>
-    <logging>
-        <log type="coverage-clover" 
-            target="build/logs/clover.xml" />
-        <log type="coverage-html" 
-            target="build/coverage" />
-    </logging>
-</phpunit>
\ No newline at end of file
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd" colors="true" convertErrorsToExceptions="true" convertNoticesToExceptions="true" convertWarningsToExceptions="true" bootstrap="test/bootstrap.php">
+  <coverage includeUncoveredFiles="true" processUncoveredFiles="true">
+    <include>
+      <directory suffix=".php">lib</directory>
+    </include>
+    <report>
+      <clover outputFile="build/logs/clover.xml"/>
+      <html outputDirectory="build/coverage"/>
+    </report>
+  </coverage>
+  <testsuites>
+    <testsuite name="Unit">
+      <directory>test/unit</directory>
+    </testsuite>
+  </testsuites>
+  <logging/>
+</phpunit>
diff --git a/test/bootstrap.php b/test/bootstrap.php
index 95bcee8..61e0f2a 100644
--- a/test/bootstrap.php
+++ b/test/bootstrap.php
@@ -1,6 +1,6 @@
 <?php
 
-declare(strict_types = 1);
+declare(strict_types=1);
 
 define('TEST_ASSETS_DIR', __DIR__ . '/assets');
 require dirname(__DIR__) . '/vendor/autoload.php';
diff --git a/test/unit/PEMBundleTest.php b/test/unit/PEMBundleTest.php
index bd43760..e3205cf 100644
--- a/test/unit/PEMBundleTest.php
+++ b/test/unit/PEMBundleTest.php
@@ -1,10 +1,15 @@
 <?php
 
-declare(strict_types = 1);
+declare(strict_types=1);
 
+namespace Sop\CryptoEncoding\Test;
+
+use LogicException;
 use PHPUnit\Framework\TestCase;
+use RuntimeException;
 use Sop\CryptoEncoding\PEM;
 use Sop\CryptoEncoding\PEMBundle;
+use UnexpectedValueException;
 
 /**
  * @group pem
@@ -14,9 +19,9 @@
 class PEMBundleTest extends TestCase
 {
     /**
-     * @return PEMBundle
+     * @return \Sop\CryptoEncoding\PEMBundle
      */
-    public function testBundle()
+    public function testBundle(): PEMBundle
     {
         $bundle = PEMBundle::fromFile(TEST_ASSETS_DIR . '/cacert.pem');
         $this->assertInstanceOf(PEMBundle::class, $bundle);
@@ -26,9 +31,9 @@ public function testBundle()
     /**
      * @depends testBundle
      *
-     * @param PEMBundle $bundle
+     * @param \Sop\CryptoEncoding\PEMBundle $bundle
      */
-    public function testAll(PEMBundle $bundle)
+    public function testAll(PEMBundle $bundle): void
     {
         $this->assertContainsOnlyInstancesOf(PEM::class, $bundle->all());
     }
@@ -36,9 +41,9 @@ public function testAll(PEMBundle $bundle)
     /**
      * @depends testBundle
      *
-     * @param PEMBundle $bundle
+     * @param \Sop\CryptoEncoding\PEMBundle $bundle
      */
-    public function testFirst(PEMBundle $bundle)
+    public function testFirst(PEMBundle $bundle): void
     {
         $this->assertInstanceOf(PEM::class, $bundle->first());
         $this->assertEquals($bundle->all()[0], $bundle->first());
@@ -47,9 +52,9 @@ public function testFirst(PEMBundle $bundle)
     /**
      * @depends testBundle
      *
-     * @param PEMBundle $bundle
+     * @param \Sop\CryptoEncoding\PEMBundle $bundle
      */
-    public function testLast(PEMBundle $bundle)
+    public function testLast(PEMBundle $bundle): void
     {
         $this->assertInstanceOf(PEM::class, $bundle->last());
         $this->assertEquals($bundle->all()[149], $bundle->last());
@@ -58,9 +63,9 @@ public function testLast(PEMBundle $bundle)
     /**
      * @depends testBundle
      *
-     * @param PEMBundle $bundle
+     * @param \Sop\CryptoEncoding\PEMBundle $bundle
      */
-    public function testCount(PEMBundle $bundle)
+    public function testCount(PEMBundle $bundle): void
     {
         $this->assertCount(150, $bundle);
     }
@@ -68,9 +73,9 @@ public function testCount(PEMBundle $bundle)
     /**
      * @depends testBundle
      *
-     * @param PEMBundle $bundle
+     * @param \Sop\CryptoEncoding\PEMBundle $bundle
      */
-    public function testIterator(PEMBundle $bundle)
+    public function testIterator(PEMBundle $bundle): void
     {
         $values = [];
         foreach ($bundle as $pem) {
@@ -82,9 +87,9 @@ public function testIterator(PEMBundle $bundle)
     /**
      * @depends testBundle
      *
-     * @param PEMBundle $bundle
+     * @param \Sop\CryptoEncoding\PEMBundle $bundle
      */
-    public function testString(PEMBundle $bundle)
+    public function testString(PEMBundle $bundle): void
     {
         $this->assertIsString($bundle->string());
     }
@@ -92,20 +97,20 @@ public function testString(PEMBundle $bundle)
     /**
      * @depends testBundle
      *
-     * @param PEMBundle $bundle
+     * @param \Sop\CryptoEncoding\PEMBundle $bundle
      */
-    public function testToString(PEMBundle $bundle)
+    public function testToString(PEMBundle $bundle): void
     {
         $this->assertIsString(strval($bundle));
     }
 
-    public function testInvalidPEM()
+    public function testInvalidPEM(): void
     {
         $this->expectException(UnexpectedValueException::class);
         PEMBundle::fromString('invalid');
     }
 
-    public function testInvalidPEMData()
+    public function testInvalidPEMData(): void
     {
         $str = <<<'DATA'
 -----BEGIN TEST-----
@@ -116,20 +121,20 @@ public function testInvalidPEMData()
         PEMBundle::fromString($str);
     }
 
-    public function testInvalidFile()
+    public function testInvalidFile(): void
     {
         $this->expectException(RuntimeException::class);
         PEMBundle::fromFile(TEST_ASSETS_DIR . '/nonexistent');
     }
 
-    public function testFirstEmptyFail()
+    public function testFirstEmptyFail(): void
     {
         $bundle = new PEMBundle();
         $this->expectException(LogicException::class);
         $bundle->first();
     }
 
-    public function testLastEmptyFail()
+    public function testLastEmptyFail(): void
     {
         $bundle = new PEMBundle();
         $this->expectException(LogicException::class);
@@ -139,7 +144,7 @@ public function testLastEmptyFail()
     /**
      * @depends testBundle
      *
-     * @param PEMBundle $bundle
+     * @param \Sop\CryptoEncoding\PEMBundle $bundle
      */
     public function testWithPEMs(PEMBundle $bundle)
     {
diff --git a/test/unit/PEMTest.php b/test/unit/PEMTest.php
index 14121b2..757ced0 100644
--- a/test/unit/PEMTest.php
+++ b/test/unit/PEMTest.php
@@ -1,9 +1,16 @@
 <?php
 
-declare(strict_types = 1);
+declare(strict_types=1);
+
+namespace Sop\CryptoEncoding\Test;
 
 use PHPUnit\Framework\TestCase;
+use RuntimeException;
 use Sop\CryptoEncoding\PEM;
+use UnexpectedValueException;
+
+use function base64_encode;
+use function file_get_contents;
 
 /**
  * @group pem
@@ -12,7 +19,7 @@
  */
 class PEMTest extends TestCase
 {
-    public function testFromString()
+    public function testFromString(): void
     {
         $str = file_get_contents(TEST_ASSETS_DIR . '/public_key.pem');
         $pem = PEM::fromString($str);
@@ -20,7 +27,7 @@ public function testFromString()
     }
 
     /**
-     * @return PEM
+     * @return \Sop\CryptoEncoding\PEM
      */
     public function testFromFile(): PEM
     {
@@ -32,14 +39,14 @@ public function testFromFile(): PEM
     /**
      * @depends testFromFile
      *
-     * @param PEM $pem
+     * @return \Sop\CryptoEncoding\PEM
      */
-    public function testType(PEM $pem)
+    public function testType(PEM $pem): void
     {
         $this->assertEquals(PEM::TYPE_PUBLIC_KEY, $pem->type());
     }
 
-    public function testData()
+    public function testData(): void
     {
         $data = 'payload';
         $encoded = base64_encode($data);
@@ -51,13 +58,13 @@ public function testData()
         $this->assertEquals($data, PEM::fromString($str)->data());
     }
 
-    public function testInvalidPEM()
+    public function testInvalidPEM(): void
     {
         $this->expectException(UnexpectedValueException::class);
         PEM::fromString('invalid');
     }
 
-    public function testInvalidPEMData()
+    public function testInvalidPEMData(): void
     {
         $str = <<<'DATA'
 -----BEGIN TEST-----
@@ -68,7 +75,7 @@ public function testInvalidPEMData()
         PEM::fromString($str);
     }
 
-    public function testInvalidFile()
+    public function testInvalidFile(): void
     {
         $this->expectException(RuntimeException::class);
         PEM::fromFile(TEST_ASSETS_DIR . '/nonexistent');
@@ -77,9 +84,9 @@ public function testInvalidFile()
     /**
      * @depends testFromFile
      *
-     * @param PEM $pem
+     * @param \Sop\CryptoEncoding\PEM $pem
      */
-    public function testString(PEM $pem)
+    public function testString(PEM $pem): void
     {
         $this->assertIsString($pem->string());
     }
@@ -87,9 +94,9 @@ public function testString(PEM $pem)
     /**
      * @depends testFromFile
      *
-     * @param PEM $pem
+     * @param \Sop\CryptoEncoding\PEM $pem
      */
-    public function testToString(PEM $pem)
+    public function testToString(PEM $pem): void
     {
         $this->assertIsString(strval($pem));
     }